There’s one hidden color in each area except the second area.
Hub: Smack the poor old man after he’s recovered.
Area 1: Waterfall room, needs a bomb
Area 2: Ice wall room overlooking a chasm on the right side, needs one of the two fire damaging skills.
Area 2 End: Smack the snowman in the upper right hand corner next to the portal. Wow, some of these are kinda heartless, huh?
Area 3: One of the village rooms, the house in the bottom left will have it’s torches out, a Wisp will light them for you to open the door. This room tends to have a tombstone that summons a wisp, so just hold off on using it until you clear the room
Area 4: This one’s tricky. There’s a room with two large unbreakable blue tesla like devices, with circuit ruts leading from them to the top wall. Requires either judgement lightning or Knight’s charge