Just to let you know, neither "染まっている" nor "染まっていた" is the progressive form. They are rather a kind of perfect tense. ／ In a word, it is (or was) already dyed. ／ If it is being dyed right now, you usually say "染まっていく". ／ i.e. 太陽が地平線から顔を出して、今まさに空が赤く染まっていく。／ In this sentence, the color of the sky is gradually changing as the sun rises. If you want to express the nuance of "gradually", you usually don't say "染まっている" here.