I know that they taught us in school, as kids, that the primary colors are red, yellow, and blue. I wish they were able to teach what is used in the real world.
For printed material, they use the
CMYK color model, which stands for cyan, magenta, yellow, and black. The black only gets added to darken things up a bit, but the primary colors of pigment are cyan, magenta, and yellow.
On televisions and computer monitors, the primary colors are
red, green and blue because light is used instead of pigment. Having worked in the area of computer graphics for a few decades, I'm very familiar with getting the colors that I want mixing those three colors. Which doesn't help a bit for watercolor.
I have a notebook where I write down all of the useful hints I can find about watercolor painting. When I read a book about watercolors, I write down the good stuff. When I attend a class or demonstration, I write down all of the good stuff. And when I get any new watercolor box sets or tubes of paint, I paint some samples in the notepad and note how those colors mix with others. I refer back to those color tests when I need a particular color.
I really like quinacridone magenta for mixing various red, orange, violet, and purple colors because it mixes so well with others.
I can mix nearly any color I want for paintings of flowers using quinacridone magenta, antwerp blue, hookers green, and quinacridone gold. There is a discussion about this here:
http://www.artistforum.com/sitemap/t-12202.html
The best suggestion I have is to try mixing the colors that you want on an old sheet of watercolor paper (the back of some painting you don't like is good) to see what you can get. I want to know ahead of time what I'll be able to get for a painting before I've used the wrong colors and messed it up.
I've worked off and on with several sunflower paintings and still haven't quite found the right color to mix with yellow to make the shadows look right on the yellow petals. I've tried about two dozen different things on my test sheet and maybe I've come close enough.
Experiment and you'll eventually find the right colors to mix to get what you want.