The greater the contrast, the more likely a combination is to pass.
And that is one of the problems with ADA compliance on websites. Websites weren't a thing when the ADA was passed and, because Congress (U.S.) didn't make new laws to cover websites, they went around Congress and claimed the ADA applied anyway. And courts are using the accessibility guidelines for websites to determine whether a site is ADA accessible. And that's the problem: they are guidelines. Some of those guidelines contradict one another. Some of those guidelines may not improve things.
The contrast issue is one example. If you have black on white or white on black, that's the highest contrast and your site is going to pass, but as demonstrated here, that might not be the most accessible option for a lot of people.
But courts use the guidelines as if they were rules.
And then you have the problem of predatory lawyers looking for sites to sue.
It would be far better for everyone if web design software developers, web designers, accessibility device manufacturers and groups with accessibility needs all worked together to develop tools as well as guidelines to make it easy and understandable for web designers and developers and small businesses and individuals to build websites that are accessible to widest possible audience.
But instead we have a situation where you face potential lawsuits for not adhering to at times self-contradictory guidelines.