Most rulers probably don't stop to consider other realm colors when they pick their own. Others may do it intentionally.
Personally, I don't see why you would *need* an IC reason to change it. It has no effect on anything IC. But if you needed it, you could justvsay you changed your mind. Or the color clashes with the color of your new warhorse's (or mistress's(!)) hair. Or you want to make it clear that you aren't associated with the other realm.
Or, you could wage war on the other realm, and force *them* to change!