Three quarters of the square are coloured and one is blank. Which square should come next?

Every colour, and the blank quarter, moves one quarter clockwise at each step. The orange quarter goes top-left, top-right, bottom-right, so next it is bottom-left, with blue top-left, yellow top-right and the blank quarter bottom-right: (a). (b) goes back to the first square and the others mix up the order.