Smrobs' mark that she is talking about is called a Blood shoulder mark and is only seen in greys, it where the pgimentation of the mark never fades and retains the base color. And while it happens and isn't rare it isnt that common either and i'm not sure that's what your mare has, thought she is pretty cute and has some interesting colors.
Caley, that's correct. The bloody shoulder marks may change shade with the change from winter fuzz to summer slickness, but they are always there.
I would bet dollars to donuts that what is going on with both your and the OP's horses are just their base color showing through and that shading will become more and more faded as they age and disappear completely at some point.
Whereas, my guy's mark gets more obvious as he gets whiter with age and he will carry it until the day that he dies.