You may be ready to invest in a set of quality tube watercolors. There are many brands to choose from! Winsor and Newton makes a very good paint that I used for years. I've recently begun using Daniel Smith and have a few tubes of M Graham. The pigment load in any of these brands is excellent.
Don't waste your money on the Student grade watercolors. The pigment load is dismal in these paints. Sakura Koi is a decent pan watercolor set. The pigment load is better than the Prang and the price is decent. I use this set for traveling.
Here is a basic palette to get started with in the tube watercolors. Note that the purple made wiith this palette is a bit muddy. That is because the Permant Red is a warm red (tends towards orange).
Here are three colors that will extend your palette further after you've purchased Basic A and Basic B. From this point, it is subjective. I find greens more difficult to mix and this gives more options.
The Dead Palette is neutral earth colors. Here are a few examples. Yellow Ochre is missing here and is a good addition. These colors soften and neutralize the basic primary palettes and provide you opportunities to mix a more sophisticated range of color.