Which coat of arms and colours would a knight wear during battle?

I'm curious about the heraldic practices of knights during battle, particularly regarding which coat of arms and colors they would display under various circumstances. For instance, if a knight serves a count or duke who, in turn, accompanies the King of England into battle in, let's say, France, would they typically wear their own colors, those of their immediate liege, or the colors of the King of England?