A useful exercise from SketchingNowWatercolour - to look at all the colours I am likely to use and to keep track of how I mix them. For each colour, the original colour, and how to mix a darker version of it. Some colours are trickier than others, like yellows - the best way to make a darker yellow is to add another, more orangey yellow or a greener yellow. Anything else added to yellow will make it muddy.
I must practise these so I can memorise them!!