In the 'wider' world of driving...i.e., ADS/FEI, around much of the world, it is considered incorrect to use russet harness with most vehicles having other than a natural wood finish(stained or varnished wood),with russet/brown 'appointments'..i.e.,shaft wraps and trace guides on shafts,hubcap color(if painted),other painted metals(bolt heads,etc.)IME, this tradition seems to often ignored/not known(even by the SOME judges, though some WILL know them) in mini breed shows, but you would likely be docked for ignoring or violating this if showing in Driving events run under ADS and similar rules, even if not 'officially' sanctioned by ADS and similar bodies...this would include 'official' CDEs, btw.
I LOVE good russet harness, myself...it is usually, in quality leather harnesses, made of 'better' leather, since it isn't really 'dyed' as is black leather harness, so flaws in the surface that would be hidden by black dyeing cannot be allowed. One reason driving reins(lines)are also supposed to be of russet('brown')is that the color won't come off onto your gloves/clothing as it would w/ dyed black reins/lines! My very first mini horse harness was an 'original' Smuckers' 'Lite', in russet. At the time I knew NOTHNG of the 'rules', just that I liked the russet. Of course, it was for my 'everyday driving', so didn't really matter. It is still the one I use for 'everyday' for my 34" driver after 30 years!
Most russet will have brass fittings, because it is 'warm' and more suitable to the 'warmth' of the russet color...though nowadays with most higher quality harnesses, you can order it w/ brass OR 'white metal' fittings...and of course, the metal fittings on the harness AND the vehicle should match, also! These 'turnout' traditions also include: driving gloves to always be brown,and your lap robe should best be a 'match' or blend quietly with, your seat upholstery color/tone. Takes some genuine planning and thought to put together a decent turnout!