Well, aside from the fact that at least 2/3rds of the army book is god dedicated in some way, and that there are no benefits from not taking a mark (aside from the fact you can choose perhaps one extra warrior for the cost of the mark), you are allowed to.
I wouldn't recommend it though - however, there's no reason as to why you can't change the fluff to state that the MoK just represents the most vicious beserkers, the MoT represents the skilled swordsmen capable of cutting an arrow out of mid air, Slaanesh warriors have faced the toughest, and feel no fear, while Nurgle Warriors are able to dodge attacks skillfully and easily.
Just change the names around, and no need to paint them the traditional Red+Brass, Green+Bronze, Blue+Silver or Purple+Black.
I wouldn't recommend it though - however, there's no reason as to why you can't change the fluff to state that the MoK just represents the most vicious beserkers, the MoT represents the skilled swordsmen capable of cutting an arrow out of mid air, Slaanesh warriors have faced the toughest, and feel no fear, while Nurgle Warriors are able to dodge attacks skillfully and easily.
Just change the names around, and no need to paint them the traditional Red+Brass, Green+Bronze, Blue+Silver or Purple+Black.
