The colbalt blue looks really nice or a brighter pink. I am not a fan of lighter colors with buckskins since it seems to give a washed out look. Since your horse has many black points I would stay with an outfit that trims out well in black.
A girl at my stable that had a darker buckskin used to show with a darker version of periwinkle. You know it's not quite blue but it's not purple either just stuck in between...but it was somewhat bold. Most people thought ewwww but on show day she really stood out and the deep tones on her horse seemed to pop out more too.