Although it is possible to mix two colours to make a different colour (e.g. yellow = green + red) it has not been found possible to produce either red, green or blue by mixing other colours.

Therefore red, green and blue are referred to as the primary colours. When these three are mixed together you make white light.

Yellow, cyan and magenta are referred to as secondary colours, because they can be created by mixing two of the primary colours.

yellow = red + green

cyan = green + blue

magenta = red + blue

 

By mixing a primary colour and a secondary colour you can reproduce white light.

red + cyan = white,  green + magenta = white,  blue + yellow = white

You can demonstrate all these effects with a suitable projector, screen and coloured light filters (but only certain types of filter work effectively).

 

Colour sources e.g mineral pigments or organic molecules can be mixed together to make a wide variety of colours.

 

Cautionary note with reference to the visible spectrum diagram above:

An opaque object that looks cyan colour might look that for two reasons

(i) the surface reflects the green and blue frequencies of the visible spectrum

(ii) or the 'cyan' coloured wavelengths are reflected off the surface

An opaque object that looks yellow might look that for two reasons

(i) the surface reflects the green and red frequencies of the visible spectrum

(ii) or the 'yellow' coloured wavelengths are reflected off the surface

An opaque object that looks magenta might look that for two reasons

(i) the surface reflects the red and blue frequencies of the visible spectrum

(ii) or the 'magenta' (purple like) coloured wavelengths are reflected off the surface

In each case there are two definite possibilities and you can't tell which is which!

 

For more on why an object is a particular colour see Part 6

INDEX of my physics notes: The visible spectrum and colour of objects

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Keywords, phrases and learning objectives for visible light and primary and secondary colours

Know that in physics the primary colours (colors) are red, green and blue.

Know that in physics, the secondary colours are produced by mixing two of the three primary colours.