Any colour (source C) can be matched by a linear combination of three other colours (primaries eg. RGB), provided that none of those three can be matched by a combination of the other two. This is fundamental to colorimetry and is Grassman's first law of colour mixture. So a colour C can be matched by Rc units of red, Gc units of green and Bc units of blue. The units are can be measured in any form that quantifies light power.
C = Rc(R) + Gc(G) + Bc(B)A mixture of any two colours (sources C1 and C2) can be matched by linearly adding together the mixtures of any three other colours that individually match the two source colours. This is Grassman's second law of colour mixture. It can be extended to any number of source colours.
C3(C3) = C1(C1) + C2(C2) = [R1+R2](R) + [G1+G2](G) + [B1+B2](B)Colour matching persists at all luminances. This is Grassman's third law. It fails at very low light levels where rod cell vision (scoptopic) takes over from cone cell vision (photopic).
kC3(C3) = kC1(C1) + kC2(C2).The symbols in square brackets are the names of the colours, and not numerical values. The equality sign should not be used to signify an identity, in colorimetry it means a colour matching, the colour on one side of the equality looks the same as the colour on the other side.
These laws govern all aspects of additive colour work, but they apply only signals in the "linear-light" domain. They can be extended into subtractive colour work.