There is, of course ...

"She is like the *purple elephant".

Does anyone knows why it is considered as a compliment?
then he says she has a wonderful personality.

Thanks a lot

Hard to say without more context, but there is a common remark about "the elephant in the room," something it is impossible to ignore. A purple elephant, in this context might mean someone so larger-than-life, so unusual, that one cannot possibly ignore it. Or her.

I paged through a few children's poems and rhymes books on the off chance that there was a purple elephant poem. No luck. Maybe I'll write one.

thank you and if you do write let us know and read.

There is, of course the Purple Cow, product of the fertile brain of Gilette (sp?) Burgess:
I never saw a purple cow,
I hope I never see one.
But I can tell you anyhow,
I'd rather see than be one.

The little rhyme became wildly popular, which caused Mr. Burgess to pen a follow-up ode:

Ah yes, I wrote the Purple Cow,
I'm sorry now I wrote it.
But I can tell you anyhow,
I'll kill you if you quote it.

Mr. Burgess was also the coiner of two words in the English language, including blurb, a wonderful word that needed to be invented, and bromide (short for bromidiom) somewhat less successful. Nevertheless, those two (out of a whole book full) took root.

That's Gelett Burgess, of course.