DS is 2y4m and doesn't seem to understand the concept of colour. He knows the names of colours and will answer with a colour when we're asking him "What colour is...?" but more often than not, it's red. So for example I eventually got him to understand his T-shirt was yellow, but when I held up an almost identical T-shirt and asked, the answer was "red".
Is this OK at this age, and when does it become worrying? Other than that I have no worries - he knows the names of most things, concepts such as turning things upside down, knows when he has one thing or two things...