Any combo top/bottom of grey with white, black, or navy would go nicely. Or any combo of white and black or grey and black.
I got white breeches free in a batch of second hand riding clothes I bought off a college girl who quit riding. She had come off her horse in her brand new white breeches on the very last time she did a 3 day event, and considered them cursed I think.
I kept them for ten years before I needed them.
I picked up a nice black synthetic summer weight blazer with 2 back vents at Value Village and changed the buttons. Then I picked up a gorgeous old school black wool hunt coat at a closeout sale 75% off and wore it to work with tailored pants for years before I got a chance to compete in it.
There’s no need to match the pad to anything. White looks fine on a chestnut.
I think it’s easy to get led astray by the vogue for matchy matchy high end gear in really gorgeous fashion forward colors, like PS of Sweden and LeMieux. The matching pad, polos, sweater and bonnet. But all this beauty is for schooling or lessons or maybe clinics. No one ever rides in hunters like this (navy or black jacket + beige breeches), and until the past year or two, no one could ride in dressage either in Paprika or Petrol Blue or Autumn Sunshine or whatever.
After all we spend more time schooling than showing.
Eventers like wild race colors for the cross country phase, of course.
My impression is that in Canada it’s still very much black & white for low level dressage because people have the gear already, and it looks traditional. I would say swapping out a very light colored breech like pale grey or beige wouldn’t stand out much. But the dark jacket and light pants is a style that cuts across English sport horse disciplines. So I think you would blend in more with beige breeches and black jacket than with a pale grey jacket and black breeches. The pale breeches also pop against the dark saddle and help the judge see what’s happening.