It doesn’t matter 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 are never going to produce a really successful painting.
The first essential is to get hold of a colour wheel and look at it carefully. First find the three primary colours, red, yellow and blue. We shall start with red. A red colour can be bluish in tone (moving towards the blue on the colour wheel) – Permanent Rose, or it can yellowish in tone ( moving towards yellow on the colour wheel) – Cadmium Red. The same applies to the other primary colours, hence a blue can be greenish in tone – Prussian Blue, or it can be reddish in tone. This is important because if you mix a reddish blue with a bluish red you get great purples.
BUT and it is a big but! if you mix a yellowish red with a greenish blue you will make a brownish red, because you areeffectively mixing the three primary colours together.
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.