Hopeful_Ink_Hoof 
I think that Autumn Harvest would work a little better as a name than Rainbow. It would let the nickname “Harvey” stay in use, but is more fitting with his color scheme. That said, I do like the appearance of the colt. It looks like he could be related to them without it being just a mix of colors. The dark colors and orange tints make him very much look like an autumn themed pegasus, and I could easily see him working a farm or the weather.