purple would look good on Rio, maybe light blue for Velvet? Bay's look good in any strong, bright colors, or "jewel tones" I think a dark purple would stand out nicely. I have purple stuff for my bay mare (+ mom's bay gelding) and they both look great in it. I ride in bright yellow mostly for my mare, but you don't like yellow =P
Velvet is a bit harder, because of the two colors. We have a similar looking horse at our barn and someone dressed him up in baby blue and it looked really nice. If not baby blue, then pick a color to compliment both of his colors.
I don't know much about western showing, but in english your mostly expected to use white. Unless you show jump, in which case go for some color! Which english discipline are you doing?
Hope this helps =)