It is very useful to know whether a colour is warm or cool when mixing paint. There are a number of ways of working out this out. Firstly, these days manufacturers of artist quality paint will often label colours red shade, green or blue shade eg: Phthalo Blue red shade. Assuming you haven’t got these sort of paints there are other ways of doing it.
1. Buy a large colour wheel which has a range of tones of green between the yellow and blue, look at your green paint and see if sits somewhere between the blue and yellow. If it does it is likely to be a cooler green – blue shade. If the green is more earthy in appearance it will probably be a red shade green (warm) as greens can be reduced in intensity if mixed with a little red. Repeat for all your colours.
2. The second method involves colour mixing and a good eye. Generally speaking cool colours are more to the blue end of the spectrum, warm colours towards the red. Therefore if you mix lemon yellow with ultramarine you end up with a pure green, because lemon yellow is a cool yellow. (you are mixing here a cool yellow and a pure blue). If you now mix cadmium yellow deep with ultramarine the green is much duller, more earthy and less strong. This is because cadmium yellow deep is a warm yellow – nearer the red end of the spectrum. You are essentially mixing yellow, blue and a little red together which would end up being brown if the balance of all colours was equal.
In summary mix your primary colours together equally and see if they produce pure secondary colours, if the do the colours are either both warm or both cool. If they produce a less pure brownish sort of colour you know you are mixing a cool and warm colour together. A process of elimination will determine which is cool or warm.
Work with only a limited range of colours get to know them and how they mix together and memorise their colour combinations and you’ve cracked it!!