A, greenhouse watermelon mainly provides suitable temperature for watermelon growth, and there is no transgenic technology, so it does not meet the meaning of the question.

B, seedless watermelon is triploid, seedless watermelon is cultivated according to the principle of chromosome variation, so it does not meet the question.

C, colored cotton is a transformed colored cotton gene, which is transferred to cotton, so that cotton cells have this colored cotton gene, and people need colored cotton, so it meets the question.

D, space pepper refers to the pepper seeds that travel in space. Because of the change of environment, their genetic genes have mutated and new varieties have been obtained, so they do not meet the problem.

So choose: C.