Wikipedia wrote:
Rouge, also called blush, is a cosmetic typically used by women to redden the cheeks so as to provide a more youthful appearance, and to emphasise the cheekbones.
I think I've found your problem:
Rouge is used on a
living person to help them look younger, however since a Death Knight is by definition
dead you should probably be using different colours to help them better fit in with main stream society.
In addition, I would suggest that applying cosmetics would be easier if you were to ask for space to set up in one of the barber shops in major cities rather than working in a combat environment with someone who will typically be wearing a plate helm.