The Stroop effect, named after Ridley Stroop, is an optical illusion. This optical illusion works by tricking your brain. Your brain reads the words and associates it with the color it spells out. The challenge is to read the word as the color the text is, rather than the word. Your brain wants to say the actual word rather than the color.
DiSpezio, Michael A. Optical Illusion Magic: Visual Tricks & Amusements. New York: Sterling Pub., 1999. Print.