There's no white variant. Posey's hair fades, and her light pink symbol paint can also fade.
The symbol paint thing can also happen to Peachy, Heart Throb, TAF Fifi . . . any pony with that light pink symbol paint, basically. It doesn't fade out as easily Posey's hair, but it can happen with enough sun exposure.
Hope that helps.
There IS a UK variant of Posey, she has light pink hair (as usual) and dark pink flowers. She is the NSS counterpart of So-Soft Posey; So-Soft Posey was given dark pink flowers to make them more visible under her flocking.
I've never heard of a UK purple flowered Posey. IMO it's probably a case of someone interpreting the dark pink symbol as purple.
Here's a picture of NSS UK variant Posey. (However, the UK also got "regular" Posey with light pink tulips.) Her hair has faded, it was originally light pink.