Sweet! I've got a Snow Onyx/Snowflake pair that both have purple/grey tail stripes. I've had 2 others here with the same thing. One of those has a bit of green in the middle white stripe. I know a local who has a female very similar to yours laying with it and I've seen another breeder that has produced 3-4 of them. They are definitely rare. I heard from one breeder that it could be caused by tank conditions early in their lives but my intuition tells me that some are just genetic and not environmental. Try and raise those eggs!