Those are great questions @ValentinaMarie. Regarding the days of the week and color correspondences. The color associations serve the purpose to help strenghten your will during a specific work, so I suggest that you use whichever color makes sense to you.
Sometimes I tend to follow my own synesthesia (when my senses blend together) to come up with correspondences on the spot. Other times I check books on correspondences to have a starting point. And then there is color psychology which is supposed to be universal.
In the case of yellow/purple, I personally would embrace BOTH because on the color wheel, purple and yellow are opposites, which makes them complementary:
