Understanding colour is essential whether you are using watercolour, acrylic, oil or pastel. if you do not understand your colours and what happens when you mix them together, you’ll never going to produce a really successful painting.
First off get hold of a colour wheel. Now lets get to know what it does. First find the three primary colours, red, yellow and blue. We shall start with red, red tones moving towards the blue will create great purples when mixed with a red moving towards the blue. In other words colours such as crimson (red moving towards blue) mixed with ultramarine (blue moving towards red) will make a great purple. BUT and it is a big but! A red moving towards the yellow mixed with ultramarine will make a brownish red, because you are mixing the three primary colours together. Checkout my video: Everything You Need to Know about Colour for more information.
This is explained easier if you look at this page of the Winsor and Newton site, it contains a colour chart of artist’s watercolours, but notice the colours are arranged in order, so the warm reds (contain a little yellow) come after the yellows, so you can clearly see which reds move towards yellow and which move towards blue.
I would suggest buying a warm and cool version of each of the primary colours, together with perhaps burnt sienna, burnt umber and white (acrylic and oil) and restrict yourself to these colours. Then learn what happens when you mix them together, so that it becomes second nature. It will be amazing how much your paintings will improve.