There are many ways to think about mixing paint, and knowing more than one is useful. I might even go so far as to say it's necessary to have multiple mental approaches to mixing paint, and have the ability to switch between them as the situation calls for it. However, here's a good method to get started:
- Choose two colors that "bracket" the hue you are trying to hit- meaning, they are on either side of the hue on the hue wheel. Occasionally one color will hit the hue exactly, but chances are it will lie between two.
- Use white to raise both of these colors to the value you're trying to hit.
- Mix the two lightened mixtures together until they match the hue you are trying to hit more precisely.
- If the chroma (better word for saturation when painting) is too high, mix black and white together to the value of the mixture.
- Use this gray mixture to bring the chroma down to the chroma you're trying to hit.
The basic idea is- start with an approximation of the hue, then match value, then match hue more precisely, then match chroma.
Of course, there are complications to this method. The biggest one is that, as written, the hue will likely shift towards blue-purple as you mix if you're using Ivory Black and Titanium white, so you would need to learn to compensate for that. One way is to choose the initial colors slightly shifted away from blue. Another way is to use a more neutral black and white- for example, Cremnitz white and certain brands of Mars black will shift the hues less significantly. You can also use a pre-mixed neutral of black, white, and brown. Note than none of these are foolproof, but they will get you closer. In the end you still need to rely on your own judgment rather than leaving it to the system.
You also need to use some judgment about the chromas of the initial colors you choose- they need to be at least slightly higher in chroma than the color you're trying to hit.
This method can also be a bit slow when painting from the model. As you get used to it, there are lots of shortcuts you can take, or you can use a different method altogether. A few shortcuts:
- Premix neutrals from black to white in equal steps, which saves time for steps 4 & 5 and also give you "targets" to match the colors to.
- Also premix the colors to the same values, which saves time in steps 1 & 2.
- Be less rigid about the order
- Adjust chroma by choosing initial colors that are further away from each other on the hue wheel (this can be unpredictable, but with experience you can learn to control this).
- Supplement this method with another method of mixing color.
It's not a magic bullet, but it's a good starting point. It's especially useful if you're used to thinking in HSB on the computer.
I highly recommend http://huevaluechroma.com, a site put together by David Briggs, who teaches out in Australia. It can read like rocket science, but it's good to read and then re-read as you work more. If you do that, more and more of the complicated stuff will make sense. There's a lot of misinformation about color out there, but huevaluechroma.com is all solid, and refreshingly free of bias.