Welcome to Librarium Online!
I cannot seem to find in the army book where/if it states whether or not a character with a mark can join a unit with a different mark.
I need to know because I was planning on putting my MoK Hero in with Nurgle knights holding the banner of rage if this is possible.
Sorry if this has been asked before but I did a ton of searching and all I could find were the rules from the CSM 'dex.
I have always played as if it were allowed. Usually I would say, "If the rules do not say it is allowed, don't do it." in this case, however, the precedent is set in the Gifts of Chaos section where is says you can take gifts of different gods on the same character. Naturally, the character can only receive the benefit if the mark he wears, not the unit he joins, but it should be entirely possible to join that unit.
I think in this case if it was dissalowed, it would be specifically stated, as is the case with the chariot. Go for it!