Why is it so hard to say the ink color when the word spells a different color?
Try to say the color of the ink, not the word, and your mouth keeps blurting the wrong answer.
The Stroop effect is the delay you feel when you try to name the ink color of a word that spells a different color - the word "BLUE" printed in red ink. Reading is far more practiced and automatic than naming colors, so your brain processes the word's meaning faster than it can produce the color. The automatic reading response and the color response you actually want end up competing, and you have to suppress the stronger one to get the right answer out. That competition is what slows you down and trips your tongue.
Take the puzzle where color words are printed in mismatched ink - "RED" in green, "GREEN" in blue - and try to say only the ink colors out loud. Almost everyone slows right down and stumbles, because the reading runs ahead of the color.
Reading is automatic - you cannot fully switch it off, so a mismatched color word will always slow you down.
It shows that a deeply practiced response runs whether you want it to or not, which is why reflexes and old habits are so hard to hold back in the moment.
The word's meaning gets there first and fights the color you want to say.
Learn the idea and practice English at the same time.