Under the Branding preview there’s a short list of contrast checks: your button labels, your links, text on the tinted panel, and the focus outline.
The plugin adjusts most of these for you
If your brand color is too light for white text, the plugin uses dark text on your buttons instead. If it’s too light to read as a link, it darkens the link text just enough. That’s why those rows usually pass whatever color you choose, and why your button labels might turn dark on a pale color. That’s on purpose, so the label stays readable.
The one that can fail
One row can genuinely fail: your brand color as a button shape against the white page. A very pale color makes a button whose edge is hard to see, even though its label is perfectly readable. When that happens, the check says so and gives you the number.
It’s a warning, not a wall
Save works either way. Your brand is yours, and a pale brand color is a legitimate choice. The check is there so it’s a choice rather than a surprise. If you want to fix it, a slightly deeper shade of the same color usually clears it.