Yes, sometimes it is just a genetic thing. Depending on genetics will play a factor in how quickly they lighten. Some horses roan out very quickly and then don't seem to change after that. Some horses grow progressivly whiter over time. I have not seen one that goes completely white. One of my horses is getting roan patching. And actually has more white hairs on one side of his body than the other. I always joke that he is half roan
. For him, I usually notice more white hairs when he grows his winter coat and again when he sheds in the spring.