I usually work with primary colours of red, yellow and blue with the addition of black and white and get by fine with just these colours when painting. It is also a good way to introduce yourself to a new medium as you can usually buy a set of these colours in a pack. All my gouache artworks are made up of primary colours that came in a pack together. The exception is that green was also included? Gouache set did. Acrylic did not. This was also the case for my acrylic paintings, until recently and got some colours that are pre-made. It does add a bit of variety, but the majority of colours you can create can all be created from primary colours. So for those of you, who just want to try out a different paint media and do not want to spend a lot of money, just get a pack of primary colours which usually include white and black and you can play around with the new medium before investing in different colours.
Ok onto some basic mixing- you may skip this if it is obvious to you to the next section. Red and yellow mixed together equals orange, red and blue mixed together equals purple, yellow and blue mixed together equals green. Add the black or white and you end up with darker or lighter shades. Mix black and white together and you have grey. Mix all three together and you get brown. You can achieve a whole range of colours light and dark with the three primary colours.
Your primary colours create secondary colours and your secondary colours mixed with your primary colours make tertiary colours. You can lighten or darken each colour with white or black. Pastels are normally mixed with white or grey to create soft colours. You can also lighten colours by mixing in the lighter shade of that mix. For example orange can be made lighter by adding more yellow or darker using red. Adding a tiny amount of black can also darken colours and a small amount of white can also lighten colours.
Hope you have found this basic colour mixing guide useful.