Why coloured paints mix to black sludge – but light colours add up to white!?

Untitled By Emma, on Flickr
“I’ve had a bit of a problem when explaining colors to my kids who have started to ask tricky questions such as ‘what colour do you get if you mix pink & green?’ Now as I understand it, white is the result of all the colours mixed together, however when you mix paint with the kids, it always comes out black.
How do I explain this to my 5 & 3 year olds?”

Fully explaining the science of paint colour mixing to a three and a five year old kid is probably a big ask. One of the best ways to answer a question like ‘what colour do you get if you mix pink and green?’ is to suggest that they give it a go. Experimenting is a great way to explore the natural world, as well as encouraging an inquisitive mindset in youngsters… and their parents!

I’m also guessing that you’re a bit baffled by why paint mixing doesn’t produce the same results as mixing coloured lights. So let’s start with light: We know that visible white light contains ‘all the colours of the rainbow’ – and that by using a prism, we can split light to reveal these colours. (If you’ve never seen it, click here). The colour of light is set by its wavelength – of all the colours we can see, red light has the longest wavelength, blue light the shortest, while green and yellow have wavelengths somewhere in the middle.

But as well as splitting white light into its different parts, we can also create perfectly good white light by mixing red, green and blue light together. This happens in TV images, white LEDs and fluorescent lamps. On TVs, monitors and smartphone screens, each pixel is actually produced from three tiny dots of red, green and blue light. So, when you are looking at a red tomato on a TV screen, it looks red because the TV pixels in that area are only emitting the red light – the blue and green dots are off. But when you look at a yellow lemon on the screen, you are actually seeing a mixture of red and green light – both red and green ‘subpixels’ are on; convincing our eyes that we are seeing something yellow! A very clever illusion.

Now suppose that we daub a blob of red paint onto a piece of white paper. The plain paper starts off white because it reflects all the ‘colours of the rainbow’. The paint blob we add looks red because it is actually absorbing all the blue and the green light, leaving only the red light to be reflected toward our eyes. Similarly a daub of blue paint on the paper will absorb the red and green light, reflecting just the blue on to our eyes. This is called subtractive colouring

So what happens if we paint the red over the blue?  Well, the red pigment absorbs the blue light and the green light. The blue absorbs red and green and so, between the two of them the two pigments account for all the light, leaving none to be reflected on to our eyes. Result: no light is reflected, and the patch looks black.

This process works in a different way to mixing coloured lights, where the idea is that colours are made by adding light of different wavelengths together. With paint mixing, you are doing the opposite, and subtracting the colours of light that the paint absorbs from white. Not surprisingly, if you mix lots of colours of paint together you subtract pretty much all the colours from white and end up with a sludgy brown or black. The knack with colour mixing is to combine the right pigments in the right proportions to leave just the right mix of wavelengths reflected to create the colour you want. (Experimenting is still the key!)

We are often told in our art lessons that there are three ‘primary colours’; red, yellow and blue whereas in our science lessons we are told that it’s different (red, green and blue as in light); and that we can use these colours to mix all the others. This, as it turns out, isn’t true: for any combination of three ‘primary’ colours there will always be hues we can see that can’t be created by mixing just these three colours. This is why, for example, high quality photo printers use six inks, not just the four that ordinary printers get by with.


Answer By Richard Ellam.

Image Source: Untitled By Emma, on Flickr



Article by Richard Ellam

September 15, 2014

Richard Ellam is a freelance science communicator, writer and historian specialising in engineering and physics. He designs and makes interactive exhibits and show props in a warm, well equipped workshop in the West of England, and also presents science shows all over the country. (see www.lminteractive.co.uk for more!)

Back To Top

One thought on “Why coloured paints mix to black sludge – but light colours add up to white!?”

Leave Your Comments

Your email address will not be published. Required fields are marked *