What are the additive primaries?

Additivity is not a special property of any particular set of three primaries. The range of colours that can be matched with any three primaries is called the gamut of those primaries.

It turns out that no three real primaries can be chosen so that their gamut includes all possible colours. If the primaries are chosen to be red, green, and blue, however, a very large number of colours can be matched. Red, green, and blue are therefore usually the colours of the primaries in an additive colour reproduction system such as colour television.