# The Naked Scientists Forum

### Author Topic: How to make colour.  (Read 4380 times)

#### paul.fr

• Guest
##### How to make colour.
« on: 23/02/2007 20:17:11 »
This may sound realy simple, and have a straighforward answer, but i will ask any way.
How are the colours white and black made? say in paint mixing, for example.

Is it simply mix equal quantities of red, gree and blue to make white? also do the different colours have to be mixed in a certain order?

#### another_someone

• Guest
##### How to make colour.
« Reply #1 on: 24/02/2007 01:46:12 »
Colour is not about what is on the paper, or on the screen, but what is in your eye.

Your eyes cannot see the full spectrum of colour, but only what approximates to red, green, and blue.  Your eyes cannot see magenta, but sees equal amounts of of red and blue, and assumes it is generated by a colour that is mid way between red and blue, and that colour is what we see as magenta.  When we create artificial colours, we try and fool the eyes (e.g. when you see magenta on a computer screen, you are really being fed equal amounts of red and blue, but the eye and brain assumes that is it actually seeing only a single colour that is midway between red and blue, so it assumes it is really seeing magenta on the computer screen, but it is being fooled into thinking that).

Paint (and printing) is different from screen colours, because what you see on the screen is created by emitted light, while paint and print relies on reflected colours.  This has a significance because on the screen, you can add colours by emitting more light of different colours, whereas when you are relying on reflected colour, you can only take away colour from what is already there.  This is why, when you add all 3 primary colour on a computer screen, you get white (and in order to get black, you must emit no light, and so no colours are added), whereas on the printed page, when you add all the colours you get black (because each colour you add, really just takes away more colour, until you are left with nothing being reflected at all).

The difference between reflected and emitted light also means that the colours regarded as primary colours for the different media are different.  On a computer screen (or TV), because we use emitted light, and create colour by addition of primary colour, the colours that are primary are those that closely relate to the colours the eyes see, i.e. approximately red, green, and blue.  For printed or painted colour, because we rely on reflected colour, and we create new colours by subtraction, the colours that are primary to the process are the complement of the colours our eyes see (i.e. most cheap computer printers will use cyan, yellow, and magenta, as their primary colours – expensive printers will also include inks, mostly lighter versions of the colours so that when the colours become mixed they do not become too dark).

In theory, the order in which the colours are applied should not matter, but in practice some inks, paints, dyes, etc., might impose restrictions that make the order significant (e.g. if a dye does not reflect a pure colour, or if there might be a chemical reaction between the dyes, etc.).  As for colours on a computer (or TV) screen, they are not actually overlaid anyway – all that is happening is that the colours are shown side by side, but so closely packed that the human eye at normal viewing distance cannot separately see the different blobs of red, green, and blue, light and so sees them and a single merged colour.  This is also what happens in half tone printing that is used in cheap newsprint.
« Last Edit: 24/02/2007 02:14:22 by another_someone »

#### paul.fr

• Guest
##### How to make colour.
« Reply #2 on: 24/02/2007 05:02:18 »
A very nice reply. Far more intresting and informative than i expected, THANK YOU.

#### The Naked Scientists Forum

##### How to make colour.
« Reply #2 on: 24/02/2007 05:02:18 »