Looking at the supplied picture of the blue lantern, er, indigo lantern, got me thinking. Indigo and blue are both pretty much the same color. When the PCs look the faded fresco in room 6, would they even notice such a subtle difference? How to describe it?
DM: "Clockwise, the colors are red, orange, yellow, green, blue, indigo, and violet"
Player: "What's indigo?"
DM: "Somewhere between purple and blue."
Player: "But that's violet."
DM: "Uh, this violet is more purplish, it's a purplish-bluish-purple. The indigo lantern is more bluish, it's bluish-purplish-blue."
Player: "So what type of blue is the blue lantern?"
DM: "Bluish-bluish-blue."