This peculiar effect is known as "Stroop Effect" named after J. Ridley Stroop who discovered this in 1930.
Why is it difficult to say the colour ?
The words themselves have a strong influence over your ability to say the colour. The interference between the different information (what the words say and the colour of the words) your brain receives causes a problem. There are two theories that may explain the Stroop effect:
- Speed of Processing Theory: the interference occurs because words are read faster than colours are named.
- Selective Attention Theory: the interference occurs because naming colours requires more attention than reading words.
I think that this puzzle would be easier for a very young child than for older children or adults. Try this out on some small kids who know their colours, but cannot yet read! I would imagine that the children would not get confused by this puzzle because the words would not have any meaning to them. They just recognise the colour not the words. (Your kid should recognise the colours.)