The Stroop Effect

A phenomenon in which individuals take longer to name the color of words printed in a non-matching color, such as the word blue printed in red ink, than when the words are printed in the same color as the word designates, such as the word blue printed in blue ink.

With the following video, try saying the color of the word, not the word that's spelled.

If the printed word and color matched, you would likely be able to say the color much faster. This is because reading words is more automatic than naming colors.

More on the Stroop effect here