In psychology, the Stroop effect is a demonstration of the reaction time of a task. When a word such as blue, green, red, etc. is printed in a color differing from the color expressed by the word's semantic meaning (e.g. the word "red" printed in blue ink), naming the color of the word takes longer and is more prone to errors than when the meaning of the word is congruent with its ink color.
—quoted from Wikipedia.
Here's a script to administer the Stroop Task to experiment participants. It will show the name of a random color in a random color (e.g. BLUE, YELLOW). You can select the interval at which it switches to the next color, and the number of such flip-cards to show in total.