RYB is the color space the app opens in. Five sliders, Red, Yellow, Blue, White and Black, mix a color the way paint mixes on a palette, not the way lights add up on a screen: yellow and blue make green here, as they do in a jar. When a color is set some other way and the five paints cannot make it exactly, the card says Nearest mix.
What it does
With RYB chosen under Color space, each color card shows five sliders, R, Y, B, W and K. Each letter sits on a small disc of its paint. Each slider runs from 0 to 100, and the numbers are parts, as in a recipe, not percentages. Only the proportions matter: 50 red with 50 white makes the same color as 25 with 25. With every slider at 0 there is no paint at all, and the color is plain white, like bare paper.
Move a slider and the recipe is mixed at once: swatch, hex code, wheel marker and the sheet on the canvas all change together. Each track is a strip of real mixes: it runs from your recipe with none of that paint to your recipe with all 100 parts of it, everything else held. You can see what adding more blue will do before you add it.
The mixing is done by spectral.js, an open-source library by Ronald van Wijnen. It does not blend the paints’ screen colors. It gives each paint a reflectance curve, a record of how much light it reflects at each wavelength, sampled at 38 points across the visible spectrum. It turns each curve into a measure of absorption against scattering, combines those measures weighted by how much of each paint there is and how strong it is, and turns the result back into a color you can see.
When a color is set anywhere else, by the wheel, the hex field, another model’s sliders, a harmony, the sliders follow it. The app searches for the mix of the five paints closest to the new color, in whole parts out of 100, using as few paints as will do. “Closest” is measured in OKLab, a color space published by Björn Ottosson in 2020 in which equal distances look roughly equally different. If that nearest mix is not exactly the color on the card, a Nearest mix button appears under the sliders. Click it and the color becomes the mix: it shifts, usually slightly, to what paint can actually make.
A recipe you mixed by hand travels with its color. Drag a swatch onto another card, or turn the colors around with the wheel’s arrows, and the recipe goes too. The app’s starting colors are simple recipes: the sheet is one coat of black on both sides, K 100, and the ground is W 100.
Why it works this way
Mixing paint is not mixing light. Take yellow and blue in equal parts. A screen that averages the two lights shows a flat gray, #808080. A pigment mixture reflects only what both pigments reflect, and yellow and blue both reflect some green, so paint gives green. The RYB sliders are there so that choosing a color here feels like mixing one, which is how a paper artist thinks.
The recipe is kept as a recipe because it cannot be worked out afterwards. A pale pink could be red and white, or red, white and a touch of yellow, or a hundred other mixes. Rather than pretend to know which one you meant, the app keeps the recipe you mixed. When the color comes from elsewhere it solves for a recipe, the nearest and simplest, and says so. Type in the hex code of a color you once mixed 50/50 and it may come back as 48/52.
The Nearest mix note exists because five paints cannot reach every screen color. The brightest cyans, magentas and greens on a screen are beyond them, and a tool that quietly rounded them off would be teaching something false.
The three primaries are red, yellow and blue, not cyan, magenta and yellow, because the app’s harmonies are built on the painter’s wheel. The sliders and the harmony buttons teach the same lesson.
The metaphor
The metaphor is the palette, and the painter’s habit of writing a recipe on the lid of a tin so that a color can be mixed again. It holds well. Mixing is subtractive, so every added paint takes light away. White makes tints and black makes shades. Order does not matter, only proportions. Three primaries in equal parts give not black but a warm brown, the “mud” every painting student makes sooner or later.
It breaks in several honest places.
- These paints are idealized. Each one is a pure screen red, yellow or blue turned into a plausible reflectance curve, not a measured pigment such as cadmium red or ultramarine. Real pigments differ in strength, transparency and grain. That is one reason many painters keep a warm and a cool version of each primary.
- The model treats paint as an even film. There is no glazing, no texture, no change as the paint dries.
- No paint runs out, and no mix is ever wasted.
- Red, yellow and blue are a teaching tradition, not a law of nature. No three pigments can mix every color, and the three that come closest are nearer to cyan, magenta and yellow, as printers found.
A history
In art
The idea that painters need only three “simple” colors was argued by scholars in the early 1600s, among them the Flemish Jesuit Franciscus Aguilonius in 1613. By the eighteenth century red, yellow and blue were widely accepted as the painter’s triad. Moses Harris built his Natural System of Colours, printed in London in the later eighteenth century, around those three. Goethe’s Theory of Colours (1810) and Chevreul’s work on color contrast (1839) kept the triad at the center of what artists were taught. Johannes Itten, who taught color at the Bauhaus in the 1920s, published The Art of Color in 1961, with a twelve-part wheel that has yellow, red and blue at its heart. It is still the wheel many art students meet first.
Meanwhile the materials changed. In 1841 the American portrait painter John Goffe Rand patented the collapsible tin paint tube, and paint could be carried outdoors without drying out. A century later Josef Albers noted on the back of each of his Homage to the Square paintings exactly which paints he had used: a recipe written on the lid.
In printing
Jacob Christoph Le Blon, a painter and engraver born in Frankfurt in 1667, took the painter’s triad to the press. In his pamphlet Coloritto (about 1725) he wrote that red, yellow and blue could compose all colors. He engraved one mezzotint plate for each and printed them over one another. He won a royal patent in England in 1719, but his London business, The Picture Office, failed in 1725. He sometimes added a black plate, though it went against his three-color principle. His one-time assistant Jacques-Fabien Gautier d’Agoty later claimed the black plate as his own invention, a claim historians dispute. In 1868 Louis Ducos du Hauron patented a photographic color process whose prints were built from red, yellow and blue pigments. Over the following century printers moved from red and blue to magenta and cyan, which reach many more colors.
In technology
The physics behind this mixer dates from 1931, when Paul Kubelka and Franz Munk published a short paper on the optics of paint layers in the Zeitschrift für technische Physik. It describes a layer of paint by two properties, how much light it absorbs and how much it scatters. It became a working tool of the paint, textile and plastics industries. In 1958 the firm Davidson and Hemmendinger introduced COMIC, the Colorant Mixture Computer, an analogue machine that solved for the mix of dyes or paints needed to match a sample. Within a decade digital programs had replaced it, and the paint-matching machine at a hardware shop is a distant descendant.
Digital painting took longer to catch up. For decades most painting software blended colors as light, so blue and yellow brushed together made gray. In 2021 Šárka Sochorová and Ondřej Jamriška published Practical Pigment Mixing for Digital Painting, bringing Kubelka–Munk mixing to painting apps in a form fast enough for a brush. Open-source libraries such as spectral.js, which this app uses, have put the same idea into the browser.
In professional software
Most professional software mixes light, not paint: blend a blue into a yellow with an ordinary brush or gradient and the middle passes through gray. Paint-like mixing is the speciality of a few painting programs.
- Adobe Fresco. Its Live Brushes imitate oil paint and watercolor. Live oils never dry, so a later stroke can always push into the paint beneath and mix with it; live watercolors spread and bloom into the texture of the paper.
- Adobe Photoshop. The ‘Mixer Brush’, grouped with the Brush tool under
B, imitates a loaded brush on a wet canvas. ‘Wet’ sets how much paint it picks up from the canvas, ‘Load’ how much it carries, ‘Mix’ the balance between the two and ‘Flow’ how fast paint builds up. It copies the handling of paint, the smear and the pick-up, more than the chemistry of pigment. - Rebelle. This painting program by Escape Motions mixes color, in its Pro edition, with a Kubelka–Munk pigment model, the same physics behind this app’s mixer, so yellow and blue make green. Its pigment color sets go further, with named paints such as Cadmium Yellow and Prussian Blue that mix as those pigments do.
- Corel Painter. The ‘Mixer’ panel is a palette to mix on. Dab colors onto the Mixer pad, blend them with the ‘Mix Color’ tool, then sample the result or paint with it straight from the pad.
What carries over. Keep the painter’s habits: think in proportions, add black and white sparingly, and keep mixes simple to avoid mud. To test whether a program mixes like paint, brush a screen yellow into a screen blue: green means it models pigment; gray means it blends light. Beware the ‘Multiply’ blending mode in Photoshop and Illustrator too. It layers colors like transparent glazes but multiplies light, so a screen yellow over a screen blue comes out black.
Try this
- On the Front card, set every slider to 0, then Yellow to 50 and Blue to 50. You get a green, #398F54. Averaged as light, the same two colors would give gray, #808080.
- Now set Red, Yellow and Blue all to 33, with White and Black at 0. You get a brown, #803B2D, not a black. That is the painter’s mud, and the reason painters keep their mixes simple.
- Type
#FF00FF, screen magenta, into the hex field. The sliders find the nearest paint, a dusty pink of red, blue and a lot of white, and Nearest mix appears. Click it to see what the paints can actually make.
Going further
The wheel these paints belong to is in Color wheel, and the harmonies built on it are in Color harmonies. For the printer’s answer to the same problem, read CMYK: printing ink.
Books: Philip Ball, Bright Earth: The Invention of Colour; Victoria Finlay, Colour: Travels Through the Paintbox; Johannes Itten, The Art of Color.