I have recently been expanding my color knowledge, so here are some articles that interested me:
Basic Watercolor Palette: 12 essential colors
A very basic article on how to use the color index
Colours to Pick for a 12-pan Watercolour Palette
This is an article about the limited palettes for 5 different artists
An article on color mixing
Working with a Split Primary Color Palette
Everything you need to know about my watercolor palette
This person starts with a few basic colors and adds more according to their needs
Limited Palette
This guy works with 2 pigments and a white
PaintMaking
An amazing site for the geekiest of us, especially this page about the history of artists' palettes
A List of Free Color Resources and Color Theory Information for Artists
More resources, if you are interested