Say the colour of the ink, not the word. Easy rule. Now try it on the word RED. If you felt a tiny stumble before you got to "blue", you have just met one of the most replicated results in psychology.
In 1935 a researcher named John Ridley Stroop published a paper called "Studies of interference in serial verbal reactions" in the Journal of Experimental Psychology. He gave students two sheets of 100 items each. On one they named the ink colour of plain coloured squares. On the other they named the ink colour of colour words printed in a clashing ink, such as the word GREEN printed in red.
The squares took about 63 seconds on average. The clashing words took about 110 seconds, roughly 74% longer. Same eyes, same mouth, same job. The only difference was that the page now contained words that said something different.
Two rounds of ten. In round one the ink matches the word. In round two it never does. Tap the button for the ink colour. The timer runs the whole round, and a wrong tap does not advance, so mistakes cost you time too.
Reading is an automatic habit. By adulthood you have read millions of words, and seeing a word fires up its meaning whether you want it to or not. Naming an ink colour is a skill you use far less often, so it is the slower pathway. When the two answers disagree, they race to your mouth, and the faster one has to be held back while the slower one finishes. That hold-back is the delay you felt.
It is not only about speed. Stroop's own explanation was that word reading is simply much better trained than colour naming. In his third experiment, volunteers practised naming clashing ink colours for eight days. Their scores on that clashing task improved by about a third.
The effect runs in one direction. Stroop also asked people to read the clashing words aloud and ignore the ink. That barely slowed them: about 2.3 seconds over 100 words, around 6%, and not a reliable difference. The strong pathway is not bothered by the weak one.
The idea that colour naming is slower than reading was not new. James McKeen Cattell had noticed the gap in 1886 and put it down to practice. What Stroop added was the conflict: put the two side by side and measure the cost.
The Stroop task is a standard way to measure how well people can ignore a tempting, automatic response and follow an instruction instead. Researchers call that ability cognitive control. Nothing about it is a test of intelligence, and a bigger gap does not mean something is wrong with you; it is a handy way of watching attention at work. Fatigue, distraction and a noisy room can all nudge the gap around.
The lesson reaches beyond ink. Anything you have done a thousand times, from reading a sign to catching a thrown ball, runs on autopilot and will leap in uninvited. The skill is not switching the habit off. It is noticing the habit and choosing anyway.