I don't always get it right the first time... But if it all goes towards a nifty looking collage in the end, it feels slightly less like wasted time. Plus if you click to zoom in, it doubles as a free game of Spot-the-Difference (they're there, I swear)! And if that's not worth it, I don't know what is.
What the heck I was doing: I originally imagined The Lady being made of smoke. Or something.
Abandoning that idea, I arrived at the painfully obvious solution of "normal colours."