The problem I see is if you get a colored cart it may limit you in the future. If the color you choose looks odd with the color of a future horse you will be forced to either buy another cart or spend big $$ to have it repainted (or be unhappy with your turn out)
This is probably the biggest aspect I would consider, too. When we build a vehicle and pick out a color, we absolutely take the horse's color into consideration. If you use the color wheel on the Hobby Horse clothing website, it really helps you figure out what looks good with your horse. That being said, greens generally are good with almost any color horse except red and strawberry roans. Yuck! But green looks great with bay, gray, black, and sorrel! I would stick with a green that is more muted, not like lime, but kelly and darker.
I also think if you want to do ADS type driving the more classic appearance of a black or sedately colored cart is a better fit.
Reignmaker has the right idea that you want to avoid at all costs looking gaudy. If you show up in red sequins, the judge might think you are Wonder Woman.
In the carriage ring, you generally want to go for a "business formal" look. (That, too, depends on the vehicle, but is generally the case with mini vehicles.) However historically, there were simple two-wheeled Road Carts that were painted fairly bright colors, like crimson, royal, yellow, and the like. It really depends on your horse. If you have a very formal horse, like a Friesian, you don't put it to a loud vehicle, although I have seen some put to muted yellows that are very nice. If you have a loud pinto, you probably don't need a whole lot of color, but black being a very formal color is ENTIRELY inappropriate for a pinto. Pintos were historically "indian ponies" and a stately person would NEVER drive a spotted horse! In the day, even high leg markings were considered gaudy. So the color and style of your vehicle really depends on your horses' colors and styles.
You all have seen my blue Phaeton Cart. I really would have liked a nice cranberry with Alax because I think that would be SO pretty, BUT since he is black, when you put him to something with more red tones, it pulls the red out of his coat and REALLY makes him look faded.
(BTW, this vehicle would look AWFUL with a sorrel! I could probably put a bay and definitely a gray to it.)
Here is the same horse put to a vehicle with red tones:
I have seen it with other blacks as well. We have a friend that has a black Shire cross and both blue and burgundy vehicles. The burgundy vehicle almost makes the mare look like a different horse.
Myrna