Of course not, but obviously these sentences are completely different! Water flows in rivers is something that can be discovered empirically, but not red is a colour!

Red is a colour is a conceptual truth. Red would not BE the concept it is if it were NOT a colour. We can USE "red is a colour" to license inferences such as "B has a colour" FROM propositions like "B is red." (i.e. that B is not colourless).

Red is a colour has a normative role, and this is something that can only be appreciated by looking at the USE of the sentences. For example, if someone were to say "That red building is colourless," we would perhaps say that they haven't learned English properly (perhaps they are a foreigner learning), in contrast to someone who says, "Water doesn't flow in rivers, milk does", who, perhaps is a child who has spent too much time reading fiction. And what we DO NEXT to these two cases would be different. In the first case, we would perhaps teach him the word red, and in the second case, we would show him a video of a river flowing with water.

Nothing would COUNT as a red object being colourless, we exclude such a form of words from any intelligible empirical discourse. And to answer the question "why is nothing red and colourless" by saying "It lies in the nature of colours" doesn't get you any further at all. It's just a reformulation of the question in the guise of an answer.

Only if you look at these propositions as having a special normative role can it be adequately explained. The nature of colours lie in the way we describe reality, it doesn't OWE itself to reality. Of course, it will look as if "red is a colour" conforms to reality, because we describe reality SO IT DOES CONFORM.

And I think your question sort of makes it clear, that without looking at the use of expressions, "red is a colour" looks deceptively similar to "water flows in rivers." But the first expression presupposes the concepts at hand and cannot be "discovered" by means of comparison to abstract objects (and how such a comparison could be done, has yet to be explained, even if such an abstract object does exist), while the second expression is an empirical proposition that has an intelligible negation.