"Ein Mann sieht Rot."
A good question.
Most newspaper and the duden seem to write "sieht rot", in such a context. I have found a few references where it is written as "sieht Rot". This seems mostly when the article is about a soccer player getting a red card, in which case it makes sense that "Rot" is used as a noun.
In summary: without context, it is probably not a noun, but can be.
... So in summary, one shouldn't be "woot'ed" for writing it with miniscule 'r' as in 'rot'?
As mentioned above this usually means the man is very angry.
In German there is an adjective red and a noun red.
"Rot" meaning the color red. "rot" meaning the red car.
And in this case it is written with a capital R and therefore a noun, meaning the man sees some thing described as red.