I made this game; in case anyone's wondering how the scoring works: It draws your version over the original with 'difference' compositing, then downscales the result (using 'medium' quality to avoid most pixels getting skipped) and adds up the R, G & B values for every pixel in that small image. This total is then compared with the total difference for the initial state (which represents 0%).

Darker areas have the biggest initial difference, and the whole pic is darker than the initial canvas, so get a lot of paint down and pay most attention to the dark bits, you'll soon get over 70%. 85% is v good, and I think 90% is probably possible with enough skill and speed.