I use shoe polish on my best harness and believe me there's not a snowball in hell's chance of me ever touching that with anything that might run the risk of ruining it!   Believe me a $600 bridle is a positive bargain compared to a set of made to measure English Leather teams harness.

I put the polish on with a very soft brush and buff it up with another brush and then a soft cloth.   I do it after the harness has been oiled.... albeit I'm not trying to change the colour.