Choose a Hue from your color wheel and find its complement.
Paint the first box with your chosen Hue, then paint the last box with its complement.
Mix in equal parts of the two colors: paint your middle box.
Mix your Hue with a little bit of its complementary color, and then paint the second box.
Mix your Complementary color with a little bit of the original Hue, and then paint the fourth box.
Make sure that each box has an equal shift in color.
Repeat if necessary to achieve the best results.