“Why does the color that I mix on my palette look different when I place it on the canvas?”
Physical considerations like opacity and surface topography aside, this is most often due to the way that we “see” color. Our sensation of color is defined by the context in which it is experienced. In fact, we have some fascinating cells in our visual cortex that have evolved to do just that— to shape our experience of a color based on a target’s surround.
With this in mind, it is easy to see how a color mixed in one context may appear very different when placed in another. So how do we contend with this? The most effective way that I have found is to use the simplest anchors possible: material anchors. Material anchors in painting are employed in the same manner that we material anchors for drawing (you can read my paper on this here: http://anthonywaichulis.com/anchors-down/).
A material anchor is a “perceptual match” between a perceived target color and a simple baseline colorant (ideally, paint straight from a tube). For example, the darkest dark in your painting may be perceptually equivalent to the black that is on your palette. It does not require you to modify it, so it is an ideal material anchor and an ideal starting point for your painting. You can apply that material anchor like a puzzle piece that begins to build the context for more complex matches. This can be done with your lightest lights, highest chroma colors, or any match that is based on a physical gamut limitation requiring little modification. Such anchors may be used early on as reasonably certain variables that can be effectively used to “solve” for more complex mixtures. As such, a reasonably helpful heuristic here is that early context building blocks (colors) should be constructed from mixtures with fewest component parts possible (ideally even as few as one).
Understand that even early material anchors may need to be altered as the entire context of a painting begins to develop, but you can be reasonably confident in those earliest stages with a “color-comparison scaffolding” built in this manner.