Having had nothing but pretty much Appies since 1973, we have had some that never changed from the color they were born with, and some that changed dramatically. Most of them, whether fillies or colts, developed more color with age, never losing any.
Roans are born solid, so will develope more with time. I have a mare now that was born solid, as was her dam, and my mare didnt really start to color til she was three or four. We had a full sized horse that didnt color out til she was five, and a friend's mare didnt until the age of 13!!!!
As far as disposition, I love the Appies, they are smart and tough. We worked our full sized horses hard, and they outdid anything else on the ranch and still had something left. They have a LOT of stamina and I find that the Minis have some of those same great characteristics.
Of course, after having the large variety, I had to have the same in the middle (POA) and wee sized horses too.
My older stallion has always been a perfect gentleman- I've always told folks if he was a man, I would have married him, LOL. I would trust this horse with my life- really. Personalities differ, just like they do with any horse, and just like any horse, some of it depends on how they have been treated and trained in their lifetime.
I do not and will not have any grays in my herd. My neighbor has a fabulous gray mare, that has produced two LOUD colored black and white pinto foals that had stunning markings- only to have both of them gray out and lose it all. I didnt realize that she was a pinto at birth too... you would never know.
If you want outstanding color that will remain outstanding, you dont want a gray horse for breeding then. I want my spots and intense color to stay that way! As do many folks who breed for any type of 'color', rather than to fade into total white in a hurry.
Keep in mind the gray gene is something completely different than the Appy roaning gene, a silver dapple, or things of that nature. The base color (black, bay, chestnut, etc...) will still stay bright, regardless of the roaning that is coming in. Some Appies roan out a LOT, but may have spots that stay, or will still have varnish marks, dark marks on their legs, etc.... they dont just turn completely white, including the spots.