For a start I would never use a stock photo for a used item.  You take the best photo that you can.

Why would you want to have a case brought against you that the item was not as described?

The best photo to use is one that you have taken yourself.  Describe in the listing that you couldn't quite get the photo to show the true colour and suggest that it's a bit like the purple on a Cadbury's chocolate wrapper.

hmmmm.... chocolate... be back soon