The goal is to make the color of the bottom object close to the color of the top object.
Press (or press and hold) on other colors to add some of them to your working (bottom) object's color.
If you think you're done press the 'Check It!' button to compare top and bottom object colors.
You can also get a hint by pressing the 'Help' button if you're in doubt what to do.
Press the 'New Color' button to get a new random color on your top object and flush the bottom object's color to black.