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Found at ps3trophies.org and thanks to The__Noid

This trophy is likely the "hardest", as the game makes no effort in showing you how to use the tools available. Open up the tools menu, then touch the arrow above the color selections at the bottom left. Notice the "+" button to the left. Touch the "+", and HOLD IT, then touch two colors with your other hand. When you let go, the colors will mix, and the trophy will pop.

http://www.ps3trophies.org/forum/paint-park-plus/200326-pain...