Light-oil harness is a recent invention for the Western Country Pleasure class so to my knowledge only Lutke and Ozark Mountain are making them at this point. Many companies make russet harness but that's usually a dark chocolate or reddish brown color as that's what is traditional and practical. May I ask what your intended usage is? Might help us direct you further.
I got a very nicely made, reasonably priced light russet harness from Mini Express Tack last year. It fits well and the leather is really quite good. It is fairly light brown now but of course, as with any leather, will darken somewhat with cleaning/oiling. I use it for the AMHR Western classes (comes with side or overcheck) and also for my one cart in Pleasure Driving with the breeching I had made for it. http://miniexpress.com/harnesses/russet/russett-harness/prod_101.html It is a very nice harness!
I have light brown on my cart that is why I am looking for a lighter brown harness. I use it for pleasure but also for some shows. The brown harness I have now is darker and is used for marathons.
This is my harness from MiniExpress and I put the conchos on myself for western, I clean, condition, and use neatsfoot oil and of course it got darker. Still like it, very nice harness for the price, I am selling it but its one of those if I sell it great if not thats fine too. Can't complain about this harness its very well made.
Traditionally brown harness is only used on a horse hitched to a cart with natural wood, would you not be penalized in a pleasure class (other that I guess "western pleasure") for a brown harness and metal cart/wagon?