I rode in whatever as a kid. The only true wardrobe fails I remember were polyester gabardine dress pants over pantyhose (super slippy) and a tube top ( leave that to your imagination). However we did not have yoga pants in the 1970s.
As an adult I like the comfort and protection of proper breeches, preferably full seat. My yoga pants just seem too flimsy. But then I also avoid the riding tights that are yoga pants with knee patches.
Once there was a rule about could only show in breeches with belt loops precisely to rule out yoga pants?
As far as white breeches some women look fantastic in them and others don’t. Unfortunately due to the pervasive body dysmorphia in our culture, the women who look fantastic in them probably hate how they look in everything.
By and large given the same size and brand of breech on the same person I don’t think white looks worse than tan or even a dark color.
We just aren’t used to seeing white pants at all as a fashion item. We had white jeans and white summer trousers or “dress pants” in the 1970s but no white breeches! I haven’t seen anyone in white summer pants for years. Even nurses don’t wear white now!