# Color of Light Simulator - Additive & Subtractive Mixing, Afterimage

> Mix red, green and blue spotlights, stack cyan, magenta and yellow filters, and see objects under sodium, LED or tungsten light via spectra and CIE 1931.

- URL: https://qilingtools.com/en/physics/color-light
- Category: Physics Tools (https://qilingtools.com/en/physics)
- Tags: [Physics](https://qilingtools.com/en/tags/physics), [Education](https://qilingtools.com/en/tags/education)
- Last updated: 2026-10-29
- Price: Free, no sign-up
- Privacy: Runs locally in the browser; files are never uploaded

A colour lab built on real spectra. Drag three spotlights on a dark stage and watch overlaps add as linear light, so red + green really makes yellow. Stack cyan, magenta and yellow filters on white to see subtractive mixing, with the transmitted spectrum multiplied wavelength by wavelength. Pick an object (red apple, green leaf, denim, banana and more) and a light source (daylight, incandescent, white LED, fluorescent, low-pressure sodium or single-colour LEDs), and the tool computes the colour you see from the spectral product and the CIE 1931 colour-matching functions, with optional chromatic adaptation. It ends with complementary colours and an afterimage experiment.

## How to use

1. In Additive, drag the three lights, change their colours and intensity, and read the overlap colours or tap any point
2. In Subtractive, drag the filters, then choose a light source and stacked filters to see the transmitted spectrum
3. In Objects under coloured light, choose an object and a light and compare its colour and spectra with daylight
4. In Complements & afterimage, pick a colour, check its complement and dominant wavelength, then run the afterimage test

## Features

- Per-pixel linear-light RGB spotlights you can drag, recolour and dim
- Multiply-blended filters plus wavelength-by-wavelength transmission plots for several light sources
- 11 reflectance spectra × 9 light sources, integrated with the CIE 1931 2° observer at 1 nm
- Optional Bradford chromatic adaptation to show colour constancy vs a fixed camera white balance
- Light complement, RGB inverse and hue + 180° side by side, with dominant wavelength
- Timed afterimage experiment that works on touch screens

## FAQ

### Why does red light plus green light make yellow, while red and green paint make a muddy brown?

Light mixes additively: both beams reach your eye together, red excites the L cones and green the M cones, and the brain reads that combination as yellow. Adding light always makes it brighter, and equal red, green and blue give white. Paints and filters mix subtractively: each absorbs part of the spectrum and passes the rest, so a mixture only returns what all of them reflect. Red paint reflects long waves and green paint middle waves, and little is reflected by both, so the result is dark and dull. That is why printing uses cyan, magenta and yellow, which each absorb just one of red, green or blue.

### Why does a red apple look dark brown or even black under a sodium street lamp?

An object's colour comes from the light's spectrum times the object's reflectance. A low-pressure sodium lamp emits almost only 589 nm yellow light. The apple reflects little at 589 nm, and the blue and green it could reflect are missing from the light, so it returns only a little yellow and looks dark brown. A green leaf or blue denim reflects almost nothing at that wavelength and looks nearly black, which is why car colours are hard to tell apart under sodium lighting. White LEDs and fluorescent tubes both look white but have different spectra, so objects shift slightly between them: that is colour rendering.

### What does the chromatic adaptation option do?

Your eyes white-balance themselves after a while under a light, so white paper still looks white under a tungsten bulb. This is colour constancy. With the option off, the tool behaves like a camera locked to daylight white balance, and white paper under tungsten comes out orange. With it on, a Bradford transform maps the light's white point to D65, closer to what you would perceive in the room. No adaptation can recover colours that are missing from a narrow-band light.

### Why do I see cyan after staring at red and then looking at white?

Staring at red keeps the long-wavelength L cones firing until they adapt and lose sensitivity. When you then look at white, the M and S cones respond relatively more, so the brain reads the white as lacking red, which is cyan. This is a negative afterimage, and its colour is roughly the light complement of the original. For the experiment in the tool, stare at the dot for 20 seconds or more without blinking.

### Are the reflectance curves real measurements?

They are smooth curves shaped after real materials rather than measurements of particular samples. For example, the leaf has a small peak at 550 nm and a steep red edge after 700 nm, indigo denim reflects more at the blue end and in the near infrared, and the banana absorbs below 500 nm. They show the principles and trends correctly, but exact colour values will differ a little from any specific object. The D65 daylight and illuminant A curves are the standard spectra.

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