A violet here is a type of flower, not a color. It's one of "viola" group. Many of them are blue, but they come in other colors too.

if it's purple it won't rhyme. All of these types of poems are designed to rhyme with blue, so it always starts with: roses are red, violets are blue...

Roses are red, Violets are blue, Kitties are sweet, And so are you. ^^

As a noun violet refers to a type of flower. It also refers to any of a group of colors, reddish-blue in hue.

BRSwartz is 100% right ^_^