You could have students write their own color poems to go with them snowmen or you could write one as a class. For example, Red as a...., Blue as a.... or My snowman has a red ___, My Snowman has a blue ___ or My snowman's head is (insert a color word), My snowman's scarf is (insert a color word), etc. Younger writers could fill in a single word on each line while older ones can write most or all of the text.
Display a copy of the rainbow poem mounted on colored paper. Then hang all of the colorful snowmen along with the student's writing around the rainbow poem.