Choosing colors for your watercolor palette can be fun for some and frustrating for others. Keep it simple. A warm and a cool of each of the primary colors, plus 2 neutrals and 2 hard to mix colors green and purple.
For my classes, I suggest starting out with 10 colors:
Warm Red - Crimson Lake
Cool Red - Primary Red Magenta
Warm Yellow - Indian Yellow
Cool Yellow - Permanent Yellow Lemon
Warm Blue - Ultramarine Blue Deep
Cool Blue - Primary Blue Cyan
Two Neutrals - Burnt Sienna & Payne's Grey
Purple - Permanent Violet Bluish
Green - Permanent Green Deep
The brand of paint I use for my classes is MaimeriBlu, an Italian superior artist quality paint. Most of the colors are transparent. Two are semi-transparent and also sedimentary, burnt sienna and ultramarine blue deep. Different brands may have slightly different names for colors. Plus colors with the same name may appear and act differently in the various brands.