In most cases you use "in" when talking about clothing: "the bride was dressed in white" = "the bride was dressed in a white gown, with white shoes." Just using the colour is the general way of describing appearance without being too specific. "Dressed all in blue." means that person was wearing all blue clothes. By using "in (colour)", you are saying that the colour is not natural, but something superficial and changeable like clothes, or even paint. "I'd like to buy a car... something in red." "I was surprised to see she'd done her room in black." Done=decorated, in this case. Remove her black decoration work and you'd see the natural colour of the room.

You look good in black. You look terrible in green.

eg: They were all dressed in black.

in before the color? What?