You’re probably thinking that the next line is, “Violets are blue.”  But guess again.  Don’t they look more purple?  If that so perhaps the poem should go:

Roses are red,
Violets are purple,
Sugar is sweet,
So is maple syruple.

Or maybe we could go more traditional, but with a rearrangement:

Violets are blue,
Roses are red,
Would somebody please,
Plug the hole in my head.

What lines would you write?