Linked from
The 11 pages that link to Color mixing, each with the reason it gives.
Color theoryBroader topic: Its outcome depends on whether the sources emit or absorb light.
Additive colorNarrower topic: Additive mixing is one major form of the broader phenomenon.
GouacheRelated: Gouache colors can be mixed on a palette or directly on the painting surface.
Alla primaRelated: Colors can mix directly on the canvas when fresh paint is worked into wet layers.
SiennaRelated: Sienna mixes readily into muted browns, oranges, and neutral shadows.