Before starting a watercolour, I like to know exactly how my colours are going to turn out. With acrylics and gouaches, you can enjoy mixing on the palette and on paper/canvas without too much planning. Well, a basic knowledge of colour mixing helps. But with watercolours, you need to have all your paints ready before you start, so I find it useful to think out my colours in advance.
In this example, not only did I check what colours I would mix, but also how different pigments would lift, as I was planning to lift paint from my paper to create clouds.
You can see a note about checking the proper pigment for Cobalt Blue. It's PB28. The Cobalt Blue I have in my box of tricks is not a real cobalt blue pigment - it's actually ultramarine plus white. Real Cobalt Blue is bad for your health if ingested or inhaled, so a lot of manufacturers approximate it by mixing other pigments. In the end, the colours I used were Transparent Orange (PO107), Ultramarine (PB29), Ruby Rose (PV19) and what I call Dirty Yellow (Aureolin from a mixing tray where it's been polluted by other colours)