Rainbows and spectra

All the colours other than the red or green or blue are being removed, so:

Red removes Green and Blue

Green removes Blue and Red

Blue removes Red and Green.

Complementary pairs

When two coloured lights can be added together to make white, we call their colours 'a complementary pair'.

When we mix the blue and yellow lights together we also obtain white. This is because the yellow is composed of red and green light.

When we use red, green or blue filters with a white light the colour is not being added to the white light.

Use of additive primaries

A familiar use of additive primaries is in colour television.

The television screen is coated with three phosphors which glow red, green or blue when excited by an electron beam. When we watch television the colours produced mix and merge to create a full colour picture ranging at the extremes from black (no colour) to white (the three primaries mixed at maximum intensity).