A Plaice is totally smooth in both directions John while as g-force says Flounder and Dab are rough. Dab also have a pronounced curve in the lateral line just before the gills.
ID by colour is almost impossible from photographs, the angle of the picture and the effect of the flash see to that (and as said Plaice, Flounder and even Dab can all have reddish spots). Fortunately there are other ways to tell, as coddy says for Plaice the giveaway is the Klingon headgear.