My capture quest is currently 'capture a Premium Palomino Leopard horse' - and I captured a palo leo but he was SAS, argh! I think I even used a Pirate map but those things never give me anything good
The only exceptions I make are for bravery or if I have straws of my old lines I'm sentimental about and try to preserve (back from the times when W ratings weren't a thing yet)
Always look for the higher rating in a stud. A mare can only have one foal per month (two at best via embryos) so there's no reason to give her an inferior stud.
When I put the new mane/tail on my horse some spots in the lineart (head, chest, legs) went black: Click
Can you please take a screenshot? And which mane/tail options did you choose?
I chose the long mane/tail options, but I just noticed the black spots are there if I preview any mane/tail option at the groom. https://imgur.com/RhklFZS