Sometimes kids know the colour too well, but just wont speak up !
When you ask them like a question , such kids will either keep quiet or they will walk off ! They can intentionally give you wrong answers too.
Also the traditional way of teaching and asking information is boring.
I have similar problem when it comes to alphabets with my 4.5 yr old. So i went and bought wooden board with cut outs of colourful alphabets and numbers from Lifestyle.
I started out by asking him to pick up all red coloured alphabets , that I had dropped off the bed. He picked them all , just the colour asked. Next I asked him to pick only blue and so on...
Now I teach alphabets with same trick. I say a letter and he picks it out of the box of numbers, small and capital letters, and fixes it on the exact place that letter is suppose to go :)
BUT , dear, your kid is too young to even worry. Some kids absorb everything , but wont reflect it. Until, one fine day , they just blurt out everything.