Visual.ly has a nice roundup of several web-based Color picking tools, including some well-known ones and a few smaller ones tuned for specific applications.

It is impossible to create anything visual without making color decisions. And to make things harder, estimates of the total number of discernable colors range from one hundred thousand to ten million. That’s a lot to choose from. Add issues of human perception and color vision deficiency, and you could get a headache doing anything with colors, let alone building a color scale for data.

via Web-Based Color Tools | Visual.ly Blog.