The most important lesson you can learn about how to accurately mix colour is the relationship that complementary colours have with each other. No matter what medium you use, from coloured pencils through to oil paints, being able to understand the effect these colour combinations have on each other is crucial to understanding colour mixing.
When mixing colour, begin by choosing a tube colour that's closest to the shade you're trying to emulate in paint. Next, look at your subject's tone to decide whether you need to make the tube colour darker or lighter.
Then look at the saturation of the colour you're attempting to mix. You can't make the colour straight out of the tube any more vibrant, so most of the time you'll be desaturating the colour or greying it down. This is where the alchemy of complementary colours comes into its own.
Let's take two complementary colours from the colour wheel onto our palette, to experiment with what we can achieve when we put them together. Follow these steps to use complementary shades to mix colours accurately.
01. Lay down a pair of blobs
Grab two decent-sized dollops of complementary colours (make sure you choose from the best oil paints around). If you ever want your greens to look more green or your reds to look more red then put some of their complementary colour next to them to provide a striking contrast in your painting. The same goes for any of the other colours opposite one another on the colour wheel.
02. Mix a little dab
Start by mixing a little of the complementary colours in with each other to see the effect that they have on each other. Depending on what colours you're using, you'll need to judge how much to add. Some paints have higher tinting strengths, so a little dab will do. Notice how they take the punch out of each other, creating the 'greying down' effect.
03. Greying down
Taking this a step further, try greying down the red completely with the green until it becomes a 50/50 mix, then push on by adding green until it becomes the dominant colour. You can see the array of lovely rich reddish-browns and browny-greens available from just two colours. These mixes may be desaturated, but they're still packed full of colour.
Now we've pushed the red all the way through to green, try adding some other colours to the mix! I've tinted the desaturated mixes with a little Titanium White so you can see the subtlety of the colours more easily. Taking the mixes to their shadow shades I've added a little blue (present in all shadows) and balanced this off with more of each complementary colour.