Green Japanese Plum

My neighbor is looking for a specific variety of plum tree.  She says it's a Japanese variety with a greenish skin.  I've been looking online for her and the only green plums I can find are gage plums.  The closest to a green Japanese plum I've seen is Shiro, which is more yellow.

Anyone have any other ideas? 

We had a tree like that in our back yard. My folks called it a Japanese plum but they also called it a Water plum.

It tasted a lot like the Beach plums that grew wild near the California coast, but it was a much bigger tree.

The fruit was a little over an inch across, and a mixture of green and red. It had a juicy, watery sweet-tart taste.

ume plums (Prunus mume) has greenish fruits with a sort of reddish blush.  beautiful flowers, sweet tart fruit.  used to make umeboshi pickles, which happen to be exceptionally delicious.

Yes, I think ume is what you're after. They're more closely related to apricots than plums, actually.

http://en.wikipedia.org/wiki/File:Fruits_of_Japanese_plum.jpg

However, there are some varieties of Japanese-American plums, such as Elephant Heart or Satsuma, which could fit the bill, because they're sometimes greenish.

Thank you all for your help.  It was an ume that she was looking for.  We've got two on the way, via the internet.