He has a nice tail now, for an appaloosa. (crap, short, thin whispy tails) send in his tail colour and get a grey/white/smokey greyor black whisp for him if you arnt satisfyed with his tail. Put a tail bag on him and brush with "no knots" horse spray, and before shows plait his tail when its wet into about 6 plaits and with make his tail full. Be sure to use a damp tail and hair spray for after. (is he allows you) Horses are very good people trainers ;)
I think that gelding has a great tail, especially for an appy. I wouldn't bother buying a fake tail for him. I would work on ways to make his tail look fuller with out it. Braiding it while it is still wet will make it wavy and fuller looking when it is dry. And you can use hairspray to add even more body to it. Until you are ready to show, I would keep is tail braided and in a bag, so it doesn't break off and will be longer and fuller.
But that's just my opinion. Good luck with whatever you decide to do.
Horse Tail Extension
I'd go with light grey since the base of his tail is basically white. Oh, and I think appies have a special rule about tail extensions, so be mindful of that. I'm pretty sure that the fake tail must be attached to the hair to hair.
I agree with the others, mostly because you have so much time until you are showing him you can really work what you have naturally and maybe save some money and really impress people with your appy's natural tail!
Take really good care of it and keep it conditioned, braided and bagged. Maybe even use MTG on it or a supplement that improves hair growth / quality. Good luck!