Here's one take on the basic idea...instead of mixing each brushstroke from scratch, spend the first 20 minutes of a 3-hour session carefully observing the model (or still-life, or landscape) and preparing paint piles for each of the large shapes in your painting. You will regain those 20 minutes several-fold later in the session.
I was reminded of premixing recently while reading the James Gurney book, Color and Light, p 122. Gurney also offers a good discussion of premixing on his blog. Read the comments on that post, too. Good information there.
Here is a link to a description of how Sean Cheetham uses premixed piles. I like Sean's method...several piles of "mud" for the light and shadow sides of his model...never used directly, but modified to match the quality of light on the form. For Sean, improved color harmony is the major benefit.
And one more link...to a post from Underpaintings, describing how Daniel Greene premixes. Greene creates more elaborate value strings of premixes, but the idea is the same.
|Michelle with downcast eyes, 8x12", oil on linen|
I painted this portrait of Michelle using premixes for the light and shadow sides. I saved major time, but the biggest benefit was the improved color accuracy. Flesh tones are always challenging for me, but I felt this gave me a more realistic result. And I used the extra time to work up brushstroke variety and fine plane changes, all contributing to a better result.