John Ridley Stroop, in 1935, discovered that it takes longer to name the colors because of interference (read his original paper here). Fluent readers automatically recognize and process the meaning of text first. When the meaning of the text conflicts with what you are asked to say (i.e. “red” when you see BLUE), your initial thought (the meaning of the text) interferes. Your brain then needs time to suppress its first thought before it can tell your mouth to speak its second thought (the color). It generally takes twice as long to "read" the colors as to read the text. Interestingly, practice helps, but will not eliminate the effect.
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