For me, I tend to design my characters around dull color palettes. Bright and bold palettes I usually stray from as they can he harsh to look at. I, personally, am a big fan of what usually show up on horses in the real world; browns, blacks, reds, whites, and even some level of blue. When I create a character, I look at the colors that exist naturally and tweak them slightly to create something that looks pleasing.
One of my OCs goes by the name of Tie Plate. He started his life as a dulled forest green unicorn with a straw green mane. While it looked good, it bugged me later on that his palette was so bold compared to my other characters (which I designed with more natural colors). I ended up going back and modifying his colors to a greyish-blue for his coat and a chocolate brown for his mane and tail.
In the end, it's all a mater of personal opinion. If it's pleasing to you, it's bound to please most who look at it. Neon colors of the like remind me of raver OCs adorned with glowsticks.