It would be hard to match a false tail to his exact color if he has other colors mixed in. Sometimes you can find a black/brown tail with both colors in it but the easier route would be to get a black tail and use Shapley's Touch Up spray to make his natural tail match the extension.
I bought mine from here for my horse and it matches perfectly, plus I have had it for a few years and it still looks great. It matches my bay geldings tail perfectly, my trainers old black gelding really well and my old lease mare who had white framing her tail and a mixture of black, sorrel and white all throughout her tail.
Personally I don't know. I can tell you that I bought a 30-33" but I do remember having to cut it as it was too long for my little 14.2 or 3 hand Paint mare. We have had to cute my fake tail multiple times for her and place it very high because when you would back her she would rip out the fake tail. So she actually broke my loop. ): But mine still works wonderfully, I might eventually even send it in and get it fixed. Now that being said even though we cut it, my tail still looks great on my 15.1 hand paint gelding with absolutely no tail! I'm not even kidding, his is pathetic.
Just in-case, I also bought it a natural black loop style tail. I still have the receipt and I have had my tail three years and I know it will last a long time
My horse is a chestnut and his tail has a bunch of different colours in it, I ordered my tail from the tail source and they were really good at matching his colour, I got one and it didnt match the best so I sent it back along with a piece of my horses tail so it was easier for them to match.