Story about flower-painting fairies

[Moonraker]

My mum remembers it having a yellow cover, thin spine, glossy cover and girl with watering can in the picture.

Have you looked in the Cave, Julie? Click HERE and you will find most (if not all) of the short story series. You can click on each range of books, and all the covers in each series will appear. There are 896 to look at, so it might take you a while! Still, if you think it has a yellow cover, that should make it a bit simpler.