Chayim Shvarzblat notes the importance of getting quality watercolors. It will take you longer and will not yellowish or degrade over time. There are many different brands, so buying a few colors from different manufacturers and trying them to and find out which one suits the best, is a good idea. 

Avoid using white, as it is not transparent. Watercolors are conserved or darkened with a different amount of water. White color will distort the look of the watercolor.

Watercolor colors can come in the form of flattened tiles, small packs, and tubes.