I never use a palette knife on my palette. To mix a tone, I plunge my brush into the paints and use the brush to mix what I need. For example, on this nearly-fresh palette, I dipped my brush into the cobalt blue–the second blue from the bottom right–and used my brush to mix it with the burnt sienna in the upper-left. I used the resulting cool brown on a face I’m painting.
I can mix whatever tone I need using this method. If I need more of a mixture, I can reliably repeat it.
I use a palette knife only to prepare a large pile of paint. If I need A LOT of a color, one that would consume all of a color pile, then I prepare it with a palette knife on a different palette, one I can preserve over several sessions. I also use a palette knife to prepare the base tones I prepare for every palette–the piles of grays and flesh tones (burnt sienna plus white) in the center of the palette.
Once I start a painting session, I use my brush to directly mix colors. This method allows me to mix colors without interrupting the flow of my thoughts. To facilitate this approach, it’s important to arrange the colors in the same way each time. I always place the warm colors on one side (reds and yellows), on the cool ones on the opposite side (blues and greens). Additionally, I arrange each color group by value–darker to lighter.
Here’s a photo of another palette session. You can see the same general arrangement as in the photo above.
(I usually add a non-standard color or two to test and review. In the above photo, the two lightest blues and the darkest green are nonstandard–they’re not normally on my palette. In the bottom photo, the violet is an example of a nonstandard color.)
The following photo is my palette after a long session. Except for the center base tones prepared beforehand, all the color mixtures were mixed directly with my brush as I needed them.