Each round shows one of four words, RED, BLUE, GREEN or YELLOW, in one of those four ink colours. Most of the time (about 85 percent of rounds) the ink and the word are different, which is what makes it hard.

The rule above the word tells you what to match: Match word means pick the colour named by the text, Match color means pick the colour of the ink. You choose between two buttons, and only one is correct.

One mistake or one timeout ends the run, so this is a streak game rather than a lives game.

The rule changes as you play. On Easy it flips every 5 rounds with 2.5 seconds per round. On Medium it flips every 2 rounds with 1.3 seconds. On Hard the rule is chosen at random every round and you have 0.7 seconds.

Every correct answer scores a base value: 50 on Easy, 80 on Medium and 120 on Hard. A streak multiplier then applies: ×1.5 from 2 correct in a row, ×2 from 5 and ×3 from 10.

A long streak is worth far more than many short ones, because the multiplier applies to every answer once you have reached it. That makes one slip very costly.

Your best score is saved in your browser under the username you pick. If you sign in, personal bests can also sync to your account.

Say the rule to yourself before every answer. On a Match color round, deliberately ignore what the word says and look only at the ink. On a Match word round, read the word and ignore the colour entirely.

On Hard the rule can change on any round, so reread it every time instead of assuming it is the same as before.

In 1935 the psychologist John Ridley Stroop published experiments showing that people name a colour more slowly when it is printed as the name of a different colour. Reading is so automatic that the word interferes with naming the ink, and the conflict slows you down by a measurable amount.

The effect is still used in psychology to study attention and the ability to ignore distractions. This game turns it into a reflex challenge, but it is not a psychological test, and the score says nothing about your health or intelligence.

• Look at the ink first when the rule is Match color. Your eyes will read the word anyway, but you can train yourself to look through it.
• Do not wait for a perfect decision. On the faster levels, a quick guess between two buttons is better than a timeout.
• Keep your eyes on the centre and use your peripheral vision for the buttons, rather than moving your gaze back and forth.
• After a mistake, a new run starts from zero. Take a moment to reset before you press start.

The multiplier reaches ×3 after ten correct answers, so late answers are worth three times as much as early ones. That means the cost of a mistake grows with the length of your streak. The best approach is steady rather than frantic: a deliberate pace that you can maintain beats a burst of speed that ends in a miss.