Iirc dun is not an acceptable color in the PRE stud book. It does occur within the breed, but it is not accepted. My instructors PRE is a dun, but he is registered grey (dam is white grey, half sister is white grey, sire is bay). He has all the dun factors, and is even considered to maybe have a pearl gene, because at 5 - 6 years old, he is not yet greying out. Dun is accepted in the Lusitano stud book though, as well as a lot of the cream dilutes. I do not think the dilutes are accepted in the PRE stud book, but I am unsure. They didn't use to be, but if they are accepting chestnuts now.... I dunno. :-\ I've never heard of a chestnut PRE. I've always known PRE to be black or bay, and grey out. And even if they were another color, the grey gene covers it up more often then not, and they are registered as a grey regardless.
Think of it not as a failure but as a success in how not to do it.
Don't look in a horses mouth for a gift.