BrightKidz Library
Subjects

Why Paint and Light Disagree About Mixing

About 10 minutes

Every child who has ever owned a paint set has run the experiment. You mix two colours and get something interesting. You mix three and it is less interesting. You mix the lot and you get the same sad grey-brown everyone else gets.

Meanwhile a stage lighting rig points a red lamp, a green lamp and a blue lamp at the same patch of floor, and the patch goes white.

Same word, mixing. Opposite ends. This is not a case of one of them being a trick.

Paint works by taking away

A tomato looks red because its skin takes in every colour except red and sends that one back. Paint does the same job on purpose: each pigment is chosen for what it will absorb.

So think about what a second pigment does when you stir it in.

The first pigment was already removing some colours. The second one removes some more. Nothing you add can ever put a colour back — every pigment in the pot is another thing standing between the light and your eye, taking its share.

Add enough of them and between them they will absorb nearly everything. Almost nothing gets back out. And a thing that sends almost nothing back is one you already know how to name: it is dark.

Every paint you add subtracts. The pot can only get darker.

Light works by piling up

Now the lamps. A red lamp does not remove anything, because there was nothing there to remove — the floor was dark before you switched it on.

Switch on a green lamp beside it and you have not taken the red away. Both are landing. Your eye receives red and green together and reports something new: yellow.

Add blue and now all three are arriving at once, which is roughly what daylight does, and your eye reports white.

The same three ingredients, two opposite answers Two panels sit side by side, headed paint and light. Each panel holds the same three small circles stacked in a column, told apart by the mark inside them rather than by any colour: one carries an upright bar, one a flat bar and one a dot. In each panel an arrow points from that column to a single larger circle, the result of mixing all three. Under the heading paint, the result circle is filled in solid and dark. Under the heading light, the result circle is left completely empty and has short rays drawn all round it, so it is the brightest thing on the panel. Same three ingredients, opposite ends. paint light
The same three ingredients on both sides. Under paint they are three things each taking a share of the light, so the answer is the dark disc. Under light they are three things each adding, so the answer is the bright one.

Nothing was reversed and no rule was bent. One process only ever removes and the other only ever adds, so of course they finish in opposite places.

Each of these is a real outcome. Decide whether it belongs to mixing paint or to mixing light.

  • Adding a third colour makes it duller
  • Everything together gives a muddy near-black
  • The more you add, the less light comes back to your eye
  • Adding a third colour makes it brighter
  • Red and green together give yellow
  • Everything together gives white

The bit you were told that is not quite right

Most people learn that the primary colours are red, yellow and blue, and that you can mix every other colour out of them.

Try it properly and you will find you cannot. A really clean bright purple never quite arrives, and neither does a clean turquoise. Generations of children have assumed they were doing it wrong.

They were not. The three that actually work for paint are cyan (a greenish blue), magenta (a pinkish purple) and yellow. Look at the ink cartridges in a colour printer and those are the three you will find, next to black. Printers are the same subtractive job as your paint box, done by people who could not afford to be nearly right.

Red-yellow-blue is a rough version, close enough for a school paint set, and it is why your purples are disappointing.

For light the three are different again: red, green and blue. Different job, different three.

Both at once

Hold a painting up to a window and two things are happening in one go.

Light of every colour is arriving from the sky. That is the adding half, and it happened before the light ever reached the picture. Then the paint takes its share out, and only the leftovers reach you. That is the subtracting half.

Which is why the same painting looks like a different painting under a warm indoor lamp: change what arrives, and you change what is left over to be seen.

A stage has a red lamp and a green lamp shining on the same white wall, and the wall looks yellow. Someone switches the green lamp off. What happens?

Worth remembering

When you next hear that two colours make a third, ask one question first: is this something that adds or something that takes away?

Paints, inks, felt tips and dyes take away, and they head towards dark. Lamps, screens and stage lights add, and they head towards white. Get that straight and colour mixing stops being a set of rules to memorise and turns into two rules you can work out for yourself.