This fun and very addictive puzzle game is called blendoku (pretty much similar to Sudoku but with colors). I find this game very useful not only for killing time on the train but for when you are thinking of a design and you stubble upon a level that catches your interest in blendoku you could use the different color values, hues or complementary colors to help formulate a design. It is available for both apple and android devices for free. Download today!