It should certainly not matter, between black and 'natural'(meaning either stained, or even, just varnished)wood finish! I happen to like COLOR, while sticking with basically 'conservative' colors that are also most acceptable in ADS-type pleasure driving-meaning, basically, a shade(the basic color plus black) of green, blue, or red-this translates to a 'hunter' type green, a 'navy' type blue, and and 'maroon' type of red-or modest variations thereof. Really 'bright' colors, like electric blue, true red, canary yellow, and even white, or tints, like 'powder' blue, or pink
- generally aren't considered correct for the light horse breed show ring(with exceptions, I imagine, but still, overall...)though I've seen such in the mini ring, more so, years ago, before most miniature horse judges became more knowledgeable/educated. (That said, the overall excellence of the performance should be what determines placing, as long as rules are not literally being broken!) Do remember that IF a 'natural' finish wooden vehicle has russet/brown trim-meaning the shaft wraps and any other shaft 'trim pieces', the seat cover, and the color of any painted metal-the only russet('natural' leather is considered proper; if the trim is black, OR, on any painted vehicle, then black harness is proper.AND, the trim on harness and vehicle should 'match'-i.e., brass w/ brass, or stainless/chrome, w/ stainless/chrome! Most use brass for the show ring--but 'white metal' is much easier to care for!
In a GENERAL way-painted vehicles MAY be considered more formal(in the wider driving world, a lot is about the style/type of vehicle), and an 'unpainted' vehicle more 'country'-i.e.,casual use. In the miniature horse singley-hitched horse show ring, though, I'd say there is little if any difference in the 'perceived' degree of formality/informality.
My own favorite color is russet(or orange);however, that is not suitable for the competition ring, by and large, so for ALL of my painted vehicles, I've 'gone with' green, it's complementary color. When I ordered my first Jerald, I went down to the auto paint place and chose a green I LOVED, and still LOVE (and admittedly, it was a 'bit' brighter than a 'hunter' green!)-and have now had three Jeralds done in it(with russet PINSTRIPING!)-- even my obstacle cart! I have never driven a horse I didn't think it looked good with! I am an artist by training and education, and black vehicles just don't really suit me. I say--if it's feasible- LIVE A LITTLE, and enjoy some color--suitable color, to be sure, but still, color! JMHO!
Margo