Brooks leather will match the seat. Yellow will match the bottle cage. Now what? I recommend ditching the yellow bottle cage for something more neutral, and going with leather. Or at least I think that is what would look better. It is entirely subjective what anybody else would think looks better, and whether the cost of brooks leather is worth that much looking better. Didn't I see somewhere on BF a post about how to use cheaper leather to make nice bar tape?

edit: you have both on hand; I would go Brooks. It cost money, why let that money go to waste? Your bar tape is not appreciating in value...