I don’t think you imagined it, actually. I remember reading about the same thing. I believe the deck containing the EV suits that Spock entered for his TMP space walk had wall panels that were painted a light sky blue. The lighting was such, however, that it wasn’t clearly visible and it looked like the plain engineering level brushed metal. Other decks were supposed to be painted other colors but we never got a chance to see them. I’m sure there’s an article buried somewhere on MA or Ex Astris that talks about it. Just to lazy today to look it up. Long story short, I believe you’re right - that was the plan at one time.