He knew red, blue, yellow, green, occasionally some of the others. If you pointed to something and asked for him to tell you the colour, he'd have a go.
For the last few weeks if I ask I get a boiler plate answer 'I don't know mummy.'
AFAIK he's not had any sort of head bump that might have affected his vision. Everything else seems fine.
Is he just going through some toddler phase I don't know about?