For purple I’d be happy with Northern Sleet, Sleetfoot (downside terribly punny but not really any worse than Guyra and Flyra; upsides related to sleet, related to her kicking boxes), or reluctantly Edelweiss. Northern Sleet if I have to vote for just one but it’s really a coin toss over Sleetfoot. I agree with Poketrix, I think it’s a snowflake not a flower, so I think something weather-related (and sleet-related specifically for the postal creed) is best. But I’m not entirely convinced I’m right it’s not a flower. I am however certain it’s an aster not a lily if it is a flower. Lilies have a superior ovary surrounded by separate anthers with large pollen strobili, not a central inflorescence disc. So while “Snow Lily” isn’t truly awful (there are a handful of common flowers called lilies that aren’t really lilies; WoW uses the name for an Edelweiss) it’s bothersomely wrong. If it’s a flower, it’s likely an Edelweiss. I’d really like to keep this one open a while to brainstorm on a good consensus name.
Blue: Lots of good suggestions. I think out of all of them, Rainy Day is probably best. Fits the cutie mark, fits the creed.
Orange: Appointed Rounds. Don’t really like any of them including my own, but I can’t come up with anything better than that, and at least her cutie mark is round and it fits the postal creed.
Grey: Derpy Hooves, natch.
Little colt: Water Spout seems good.