In the book ‘Creative Illustration’ by Andrew Loomis there is an interesting theory about color relations.
In short, he explains that by mixing a little of one color into all the other colors on the palette, the colors become related and a harmonious painting is automatically the result.
Instead of mixing on the palette you can also paint directly into a wet underpainting. I tested out this technique and copied one of the four example portraits from the book.
I covered the whole canvas with a thin layer of blue, into which I painted all the other colors.