I See Spot has a darling line of harnesses (with matching leads!) that are reversible. Even with all of the polka dots and florals, I think my favorite would have to be the Yellow Check harness that reverses to show multi-stripes in colors reminiscent of rainbow sherbet. Just too cute!
Harnesses are $32 and leads are $16. They even have coordinated pouches for your keys or baggies for $18.