why is violet blue on this love poem?isn't it supposed to be purple?
Roses are red,
violets are blue.
The sky is beautiful,
and so are you.

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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.

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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...

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

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

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BRSwartz is 100% right ^_^