Roses are red can refer to a specific poem, or a class of poems inspired by that poem. The original poem is:

Roses are red,

Violets are blue,

Sugar is sweet,

And so are you.

Most variations on this poem are puns, as the following:

Roses are red,

Violets are purple,

Sugar is sweet,

And so is maple syruple.

However, these poems run the gamut of joke topics, and may also be somewhat crass.