I have learned (是+color+色+的)pattern to describe colors. I am told that "Color" is a quality that cannot be compared. (you can't say this is bluer than that) Because of this, one should use the 是+...+的 pattern. But recently I came across 天空很蓝:The sky is blue. Is this pattern correct for talking about colors?
In short, yes. 天空很蓝 and 天空是蓝色的 are essentially of the same meaning, and the former expression is more common in conversation. We usually do not use colors for comparison, but it is common to use them as descriptive words.
Some more details:
天空很蓝 uses 蓝 as a descriptive adjective. The emphasis is on the appearance of the sky: What does the sky look like today? It looks really blue. You can feel certain emotions from the sentence with this descriptive tone. You need to be careful about one thing though: 很 is a very important connected word in this expression. 天空蓝 would be referring to the kind of blue that is similar to the blue of the sky.
天空是蓝色的 is more like a statement. The emphasis is on the fact that the sky is of the color blue rather than other colors: What is the color of the sky? The sky is blue.
Hope that helps.
I don't know why you have been told that "cannot be compared", you might need to show some examples. Here are examples of color Comparision: "今天的天空比昨天(要/還/更)蓝" - Today's sky is bluer than yesterday. "大海的蓝是深蓝色的而天空是淡蓝(的), 所以大海比天空色深." - The color of the ocean is deep blue, the sky is light blue, so the color of the ocean is darker than the sky.
"天空很蓝" is correct, it says "the sky is very bluey", a descriptive sentence, as opposed to "天空是蓝的" that says "the sky is blue", a general statement.