My cousin was like this. Eye check up revealed he was colour blind. Match colours no prob cos it appears the same shade to them. But to identify colours, cannot.
You can try making it into a song, or point to objects and ask the colour.
Use the surrounding to 'test' if your child knows the colours. Could be she dont like the flashcards anymore so she refuse to tell you the colour also. Try with objects/toys she like or cartoons.
E.g. You ask, "What colour is Minnie's ribbon?"
"What colour is the train?"
"What colour is the car?"
"Is this blue?" (deliberately ask the wrong colour)
We taught my boy colours through flashcards first, then use surroundings to emphasis.