The usual advice for a beginner is to start by buying a small collection of basic colours and then practice mixing them to create further colours and shades. This is good advice and I would suggest that a beginner chooses a basic set of watercolours to start with. You can choose tubes or pans, it’s up to you and really a matter of personal preference, I use both.
You already probably know that with the three primary colours of red, blue and yellow you can mix the three secondary colours of orange, green and purple, so maybe you would think that you could save money and just buy a red, a blue and a yellow and mix every colour you wanted to use from these. The problem with that is which red (or blue or yellow) would you choose? Some reds have more blue in them than others, alizarin crimson for instance, while others have more yellow in them like cadmium red, so for a start do you want to purchase a warm or a cool red? The same applies to all the colours available so it is better to begin with a watercolour set that has at least twelve colours in it, then you will have a starting point from which you can mix lots of others.
If you mix red, yellow and blue together you will get black and you can alter the shade, from cool to warm, depending on the proportions of each colour you use. Adding more water will give you grey, but more interesting greys can be made using ultramarine blue and burnt umber, or burnt sienna and winsor blue. Experiment and see what a wide range of colours you can mix to use in your paintings.
A word about green
For some reason green seems to present the most problem for beginners and for quite a few more experienced artists as well. You can buy green watercolour paint, but many of the ones available are rather unrealistic if you are planning to paint a landscape for instance. Obviously you are free to paint your grass, bushes and trees any colour and they don’t even have to be green. Close observers of nature will know that any tree or the foliage of plants and flowers can display a whole range of colours, but if you do want your grass and your trees to be a more realistic green then there are a few colour combinations that I suggest you try. Winsor blue and cadmium yellow, ultramarine blue and lemon yellow and ultramarine blue and cadmium yellow all produce very natural greens. Or you can try adding another colour to your existing green paint, for instance try adding burnt umber to winsor green.
Two final watercolour mixing tips
I do find that in general artist’s quality paints mix better together than student quality ones, and often seem to have more clarity, so it may be better to buy a smaller selection of quality paints and practice your mixing, rather than starting out with a larger selection of student quality paints. However, you should buy what you can afford, don’t let the idea of using student quality paints put you off, many are very good and for some colours there is little or no difference between them.
Lastly, you may find it very convenient to buy lots of small pallets rather than using one big one. Once you have mixed your colours and found ones you like and want to use it is easy to keep on reusing the colours from the palettes, there’s no need to keep washing your pallets when you can always wet and reuse previously mixed colours again for another painting. It’s a great way to avoid waste and save mixing time as well.