Complementary colours are always opposite one another on a colour wheel.
The complementary colour of a primary colour is always a secondary colour on a colour wheel.
The pairs of primary and secondary colours that produce complementary colours depend on the colour model:
When mixing light (using an additive colour model) the pairs are red-cyan, green-magenta and blue-yellow
When mixing pigments (using a subtractive colour model) the pairs are red-green, yellow-purple and blue-orange
In the case of digital printing (using a subtractive colour model), the pairs are cyan-red, magenta-green and yellow-blue
The intermediate colours between three tertiary colours on a colour wheel always include one primary and two secondary colours. Adding the two secondary colours together produces the complementary of the primary.