A: Colour theory
Colours are described in terms of:-
Hue – the actual local colour ie: an orange leaf
Value or tone – the lightness or darkness of the hue
Intensity – how dull or bright the hue is
Temperature – whether the hue is situated on the cool or the warm side of the colour wheel and where it is within that side of the colour wheel
It is necessary to get contrast in one or more of these elements to make a colour appear brighter. I painted four “orange” leaves to illustrate this.