Drag a coloured spot anywhere on the plain white dress! And if you want to take that spot off then drag it off and place it back to where it was before!
I went on GOOGLE images and searched: 'plain white dress'. And then uploaded it as a sprite, but I also rubbed the outside off. Then I made my own sprites, the coloured spots and went on 'pen' and used the operator: 'stamp'! :)