I do not know if this is the right place to ask but I just cannot find any reference to a sept colour for Dal'yth. Is there any Canon one at all?
Well, Lexicanum (http://wh40k.lexicanum.com/wiki/Dal'yth#Sept_Summary)says the Sept colour is purple, while the uniform varies between dark green and grey. But on the same page it also says "Colours: Unknown" so that might not be reliable. Anyone got an up to date codex to look in? I only have old versions, and I don't even have them with me.
In terms of paint schemes in both thee 4th and 6th edition books. Dal'yth is not mentioned at all.
Examples on Google images show Dal'yth tau in Dark Green uniform with a Purple sept marking. Couldn't see the cloth colour that well though.