Here's a useful mnemonic for the results of mixing primary and complimentary (or secondary) colours:
Blue, Green, Red = White
BGR are the primary colours of light, also known as additive colours. When all three (BGR) are combined, there is total colour (ie. white light).
The secondary YMC colours are the primary colours of paint (used to make most other shades), called subtractive because they each absorb (or subtract) one primary colour of light (reflecting the light of the other two only). When all three (YMC) combine, there is no colour (ie. light) left to see.
The mnemonic also shows the ordered relationship between BGR and YMC:
Yellow = non-Blue (ie. combined G/R only)
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