Can anyone tell me about the colour of daphnia?

I have a small bag of it, bought an hour ago from my LFS, which is on my work desk at the moment, swimming merrily away (blissfully unaware of the fishy-fate that awaits it when I get home).

However, some of the smaller ones seem to be green in colour. Does anyone know if they are meant to vary at all? I've never really stopped to look before!