However, nothing is as good as trial and error, and experimenting on practise paper before adding a mix to the canvas.
Always mix a slightly larger quantity than you think you might need as it is often difficult to achieve the same colour mix again.
Also most useful is the colour wheel, although this is more for determining contrasting and complementary colours.
I recently came across the follow advice which is a very good guide to follow when mixing colours:
1) Decide which one of the following or combination of two or more is needed.
2) Start with the most predominant value and colour seen in the subject being painted.
3) To lighten add a colour one up on the colour wheel (eg: to red add red-orange); or white (white makes tints and can make colours opaque); or a lighter value warm or cool, same family, grey (mellows colour).
4) To darken, add a darker colour in the same colour family (eg: to ultramarine blue, add prussian blue); or a darker value complimentary colour (this will also dull the original colour); or a darker value warm or cool, same family, grey (mellows colour); or black (know the base colour – ivory is a yellow black, vine black is blue).
5) To brighten, add a more of the predominant colour in the same value; or for warm colour mixtures add the next colour up on the colour wheel; or a a brighter colour in the same family (eg: to ultramarine blue, add cobalt blue); or yellows and orange colours, if they’re not complimentary to the mixture
5) To dull a colour, add a complementary colour of the same value; or earth colours of the same value (eg: raw sienna); or a warm grey in the same value to a cool mixture (this mellows colour); or a cool grey in the same value to a warm mixture (again, this mellows the original colour).
6) To warm a colour, a warm colour in the same value; or warm earth colours in the same value. Note: colour is relative to that which it is compared, but by themselves, for instance, burnt umber is a dark yellow and burnt sienna is a dark orange); or a same value warm grey.
7) To cool a colour, add cool colours in the same value; or cool earth colours in the same value (eg: raw umber is a deep green); or same value cool grey; or cool-based black (eg: mars black is a blue black).