If it makes you feel better to wear the green coat, get it. But I wouldn't look twice if you were wearing a black coat with brown boots in nasty weather. Sometimes it has to be about practicality.
Mom to Two Wild and Crazy Boys and One Sweet Baby Girl
Neutrals are mixed all the time these days. I have actually read that it's more trendy to mix them, no idea if this is a true fashion "rule". I think it's perfectly fine. Also fine to buy the coat in a different color. Costco outerwear is so cheap you'll still be coming out ahead of shopping elsewhere. I also have had BIG regrets from not stocking up on favorite costco items. (As me how many knockoff barefoot dreams sweaters I own from costco. Ummm I have definitely purchased 6....and I think I own 4 currently.)