Aquarella HD is a really fabulous app by accomplished photo app developers JixiPix Software. It is of course a watercolour affect app – I have Aquarella pencils that I use to ‘touch up’ Polaroid prints. Aquarella essentially delivers a wonderful transparent and luminous effect to your iOS images. Usually Aquarella HD retails for $1.99/£1.49 but today it is free, click here to download.