Do judges really care what you wear in the show ring (within guidelines, of course). Like if one wears a red polo on a black horse in summer…

BILL– In the old days “everything was black and white.” Yes, very minor variations were permitted – breeches could be tan or buff. Navy blue coats were acceptable. In recent years the standards have been liberalized considerably, although an elaborate coda outlining the parameters of the FEI’s HSV color wheel is in the rules to limit riders’ flights of sartorial fancy.

From the USDF Dressage Connection, October 2015. https://www.usdf.org/EduDocs/The-Rider/Dressed_to_Thrill.pdf

In real life what does it all mean? That depends somewhat on how strictly the rules are enforced. I can think of some (dare I say “stodgy”) TDs who would be quick to write you up or even get you eliminated if you exceeded the technically defined boundaries. Others and many judges will simply look the other way.

To be on the safe side, muted shades are better. Stripes are probably best avoided unless they are narrow and very discreet. The rules are also very explicit about the size of logos or other printing on pads or on shirts if jackets have been waived.

As for the original question, at a summertime schooling show I would have no problem with an ensemble like black breeches and a red polo shirt. Some might. It’s such a matter of degree and interpretation. Forest green – yes; lime green – no. Go figure!
When I judge I try not to be grumpy, to give the rider the benefit of the doubt. At one recognized show, however, someone came in on a black horse with the most garish, electric blue saddle pad you could imagine. It was also all fuzzy like a bathmat. I let her ride, but it was incredibly distracting and not an enhancement to her performance in any way. I suggested that in her next test she might find something else for him to wear.