Just like glass colors... it depends. Generally, browns are very common. Blue shades are more collectible but they are not rare; still, there are some shapes that are extremely hard to find in blue and they can go for big bucks. Green is not common, but most greens are not particularly pretty so they don't command a high value unless they're an unusual shape. Yellow is a mixed bag... there are some that are more tan or honey colored that aren't rare or unusual. There are some bright canary yellows that are very tough. White is very common in modern stuff, but if you can find an old pre-1910 piece in white, it's quite unusual. Sky glaze is current production and generally doesn't have much collector value.