What's happening?

Even though you're only concerned with naming a word's colour, it seems impossible to ignore the word's meaning. You may see red ink - but if the word says "green", you feel strangely tongue-tied.

How come?

You've just experienced interference.

When you look at one of the words, you see both its colour and its meaning. If those two pieces of evidence are in conflict, you have to make a choice.

Interference is the result of having to choose against a stronger type of evidence.

In this case, you have to choose against word meaning, which experience has taught you is usually more important than ink colour.