Exercise 15 - Grays and neutrals from yellow and violet with green-yellow and mixed bright violet. Winsor Lemon, French Ultramarine, and Permanent Rose.
I particularly liked the colors in the center of the chart. This color set is one of my favorite exercises.
As I read through my various books on botanical watercolor, I notice that some artists work with a very limited palette, something similar to these exercises. And yet, there are others who may use ten different pigments (and some of them are already mixes!) in one painting. Everyone's work looks great. I guess it's a matter of personal preference.