I am sucker for a chestnut with chrome. Other than that, I am a big believer in a good horse is any color. I owned a fabulous horse that I believe was the ugliest App ever foaled. Cream colored, few spot, pink skin, just a few roan hairs on the legs, rat tail, no mane…
My preference for geldings is based mostly on the fact that it’s easier to manage turnout on my farmette as a single sex dorm than a coed. However, I did just buy a mare so there’s that.
Finally, I agree with the others that size will be a much bigger limitation on resale than sex or color. Size isn’t as big a deal with eventers as it is with hunters, but there’s still a bias. If you go for something under 16 hands, make sure it has a really efficient galloping stride and is comfortable jumping combinations set for a 12 and 13 foot stride.