Hmm.. Breast cancer pink would look nice with a bay gelding but A med. Blue pops out more on a Bay :) But then again it also depends on size. Different colors pop out on different sizes and coats :) Hope I helped!
I have a tri-color paint and wear either black and white, purple, or turquoise / teal on him and they all look great. I also have a coral colored rail shirt with black and white accents that looks good, too.
A little trick I discovered a couple of years ago.......
Take a good picture of your horse that shows off his color, preferably one from the side. If you see a shirt you like on the internet, copy the picture onto a Word document, then copy a picture of your horse onto that same document. It will give you an idea what the shirt or color would look like on your horse. This is really helpful if you have a Paint or an Appaloosa.