The colour wheel consists of the primary colours—red, yellow and blue, and the secondary colours—green, orange and purple (or violet).
Generally speaking, reds, oranges and yellows are warm colours and greens, blues and purples are cool colours.
Part of the difficulty in mixing watercolour paints arises from the fact that there isn't a "hue neutral" pan colour for each of the primaries–red, yellow and blue. Some are close, but most have a colour bias, or leaning, toward some other colour.
The purest, most intense mixtures come from combining two primary colours that lean toward (are "biased" toward) the same secondary colour.
The more colours you mix together, the greyer and less pure your mixtures will become. Colours that are opposite each other on the colour wheel (like red and green, for example) will also neutralise each other when mixed, and make a greyish, brownish colour. Try always to mix the colour you want using no more than three colours. Start with the lightest of the two colours, and add the darker one to it, a little at a time, until you get the result you want. Remember that watercolour dries lighter, so what you see in your palette should be a deeper, more saturated mix to compensate for this.
Experiment with some colour mixing
Here are some suggestions:
French ultramarine or ultramarine blue mixed with permanent alizarin crimson or thalo crimson. These two colours both lean toward violet, so will give you the 'purest' purple mixtures.
French ultramarine or ultramarine blue mixed with Winsor lemon. These two colours will give you good greens, but because the blue leans toward purple (has a little red in it), the greens aren't 'pure'.
Permanent alizarin crimson or thalo crimson mixed with Winsor yellow. These two colours give you the least pure, intense oranges because they lean toward completely different colours. If a quieter, duller orange is what you want, use these.
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