A theoretical "experiment" about knowledge philosophy where it asks whether you can even "understand" colour without actually experiencing it. This is because you are stuck in a greyscale room. Even if you read all the books about colour, you cannot actually "experience" colour.
"Understanding" itself is too vague a term, and is hard to define! On one hand, you at least know some stuff about colour wavelengths, but on the other hand, hands-on experience of colour is probably needed. So I would say your understanding is somewhere in the middle. Philosophy is not so black-and-white!