It is common that as you are painting, much of the paint that you apply will not be used straight from the tube, rather it will be mixed. This mixing can take place on the canvas or on the pallet. Most of the ‘serious’ mixing will be done on the pallet with the fine-tuning being done on the canvas. The science of mixing is simple enough…
First: arrange your paints in a rational arrangement around the edge of the pallet. I suggest grouping the primaries… something like: Lemmon Yellow, Cadmium Yellow, Cadmium Red, Alizarin Crimson, Prussian Blue, Cobalt Blue.
The following is a generalisation of how mixing works. We first start with a base color which we will then moderate. By mixing one or more paints, the following functions may be served:
To darken the base color.
The complimentary hue may be used for this purpose. Alternately black. Black is very useful for darkening browns (Raw Umber, Burned Sienna etc) but can also ‘kill’ a bright color like yellow. Black is also useful for making browns from ‘higher pitch’ colors e.g. a Raw Umber from Lemmon Yellow plus Black.
To lighten the base color.
White may be used for this purpose. However, this will also desaturated your color. To counter this the applied paint might need to be glazed or stumbled with the base color after it has dried. Alternately, a higher pitch hue may be mixed in (e.g. lighten a red by adding white, then add yellow to regain saturation).
To desaturated the base color.
Again, the hue opposite of the base color may serve this purpose well. Also black or white.
To change the hue of the base color.
If your pallet contains all the primaries in their warm and cool pairings, but does not contain any secondaries, then of course you will have to mix your paints in order to make them. The temperature of the primary will be important. You may see for yourself by comparing the number of different greens that can be achieved by different combinations of the warm and cool yellows and blues.
As you can see, these operations can be broken down into the hue, saturation and lightness model. You will also notice that an operation on any of these values will have an effect on one or more of the others.
Black as a darkener should be used with caution. In painting the shadows of an object you will find that black will ‘kill’ the shadow. It is a better option to use the hue opposite.
In general, you will find that in order to get what you want you will need to mix no more than three paints. It is important to keep your base colors arranged in a rational manner around your pallet.