Too bad, because European lilac exists often these three colours, so it would have been fitting. (At least, I expect to see those three blooming around in my neighbourhood in a few weeks)
With Hiragana sublegends, maybe a Japanese name would be fitting?
Lilac (tree) = "Rairakku" or "Hashidoi", with the latter maybe more associated with the indigenous variety which however does have yellowish flowers. Therefore "Hashidoi" might not be as fitting.
However, "purple lilac" = "murasaki no rairakku" or "murasaki no hashidoi".
Therefore, my suggestion would be "GMK Murasaki" ... unless that is also already taken.
