I second ASOS as a recommendation - they have the widest range I think and I bet they'd still have some more summery or colourful options either full price or in their sale.
H&M have got a smaller range but do have some dresses and blouses at the moment in colours other than black and grey (although most of their tops and trousers are those two colours!).
Dorothy Perkins have got some floral and summery printed tops, trousers and dresses for a bargain in their sale - but not sure what range of sizes they have left. Their full price range includes some nice workwear dresses, including a few prints.