How to use it
Pick a mode above the picture. Light shines red, green and blue spotlights on a dark wall, Filters lays cyan, magenta and yellow filters on white paper, Paint mixes paints on a palette and Objects puts six everyday things under a lamp you choose. The panel beside the picture shows the colour you are looking at: its nearest name, hex code, sRGB values and how strongly it stimulates the three kinds of cone cells in your eye.
Controls
- Light and Filters: drag a spotlight or filter to move it; tap anywhere to move the probe (the small cross) there. Sliders set brightness or strength. The coloured areas are labelled, and the mix table lists all combinations.
- Color match: “Start the challenge” gives a target colour to make where all three overlap, with hints for each slider.
- Paint: tap a paint on the palette, or use its + and − buttons, to add or remove one part.
- Objects: choose a lamp, then tap an object. The graph shows the lamp’s spectrum, the object’s reflectance and the reflected light.
- Keyboard: focus the picture. Keys 1–3 pick a light or filter, 4 the probe, arrows move it, + and − change it. In Paint arrows pick a paint; in Objects they pick an object and [ ] switch lamps.
How it works
Adding light
Where beams overlap, their light powers add:
Red and green make yellow, green and blue cyan, red and blue magenta, and all three white. For display, each linear channel C goes through the sRGB curve: V = 12.92·C for C ≤ 0.0031308, otherwise 1.055·C1/2.4 − 0.055, times 255.
Taking light away
In this ideal model cyan absorbs only red, magenta only green and yellow only blue. Stacked filters multiply their transmittances, T = T1 × T2 × T3, so cyan and magenta leave blue, cyan and yellow green, magenta and yellow red, and all three black.
Mixing paint
Paint particles absorb and scatter light, so the Paint mode uses the Kubelka–Munk model. At each wavelength a paint with reflectance R has K/S = (1 − R)² / 2R; the mixture’s K/S is the average weighted by parts, and it reflects
Blue and yellow paint give green because both reflect some green, while each absorbs most of what the other reflects. The paint spectra are smooth model curves, not measurements of real products.
Objects and lamps
An object’s colour is the light it reflects: the lamp’s spectrum S(λ) times the object’s reflectance ρ(λ), summed from 380 to 780 nm with the CIE 1931 2° standard observer,
and converted to sRGB with the standard matrix. Daylight is CIE illuminant D65, the bulb follows Planck’s law at 2700 K, the LEDs are narrow bands and the sodium lamp is its 589.0 and 589.6 nm lines. Each lamp is scaled so a perfect white surface shows at full brightness, like a camera’s automatic exposure; the single-wavelength lamp keeps equal power, so the spectrum’s ends look dim. Pure spectral colours lie outside what screens show, so negative sRGB values are set to zero.
Your three cones
The retina has three kinds of cone cells, most sensitive to short (about 420–440 nm), medium (534–545 nm) and long wavelengths (564–580 nm). The brain gets only these three signals, so red plus green light, which stimulates L and M cones like pure yellow light does, looks yellow. The cone values here are estimated from the CIE functions with the Hunt–Pointer–Estévez matrix.
Things to try
- Turn the blue light off: the overlap turns yellow. Lower green to 25 % and it becomes orange.
- Set all three lights to 50 %. The mix reads 188, not 128, because sRGB values are not proportional to light power.
- Stack cyan and yellow filters to get green, then make green paint from blue and yellow.
- Mix cyan, magenta and yellow paint and compare the muddy brown with “Mixed as light instead”.
- Put the tomato under the green LED: it turns very dark. Under the sodium lamp every object is a shade of orange.
Frequently asked questions
Why are the primary colours of light different from those of paint?
Lights add wavelengths; paints and filters take them away. Red, green and blue work well for adding, and their opposites cyan, magenta and yellow for subtracting.
Why does 50 % brightness show as 188 instead of 128?
The sRGB curve spends more of its 256 steps on dark shades, where our eyes notice small changes, so half the light power corresponds to 188.
Why do printers use black ink?
Real cyan, magenta and yellow inks are not perfect filters, so together they give a dark brown rather than deep black. Black ink is also cheaper and prints sharper text.
Is this exactly what my eyes would see?
Not quite. Your brain adapts to the lamp (colour constancy), so white paper still looks fairly white under a warm bulb. The page shows the raw reflected light, like a camera with a fixed daylight white balance.