They tell you that there is a straightforward standard for colour contrasts, but for thing, it’s about the contrasts — not the colours.
Forget red and green. Not everyone sees red; not everyone can tell red from green. Think: blue. Blue does not ‘have a good contrast’. Navy blue against white has a good contrast; black against mid-blue might have a good contrast; some contrasts (like white letters reversed on a deep black background) are too overwhelming for good contrast (because white is luminous). Best not confuse accessible contrasts with specific colours, when a specified colour will have standard contrast sometimes, and at other times, not.
Stare at white type on deep black for long enough and it starts to do what a rebus does — one of those images which was two white faces when last you looked but is now one black candlestick. You need to tone down the black to a 90% grey. You may need to de-saturate your mid-blue.
The main point here, however, is contrast. Contrast exists between one thing, and another. Brand colours may or may not have accessible contrasts, — depending on context. And here’s one other, historic thought.
Readers used to expect texts that were copied by hands. They used to want type that looked like handwriting. Gothic script looked as though it had been made with the triangular nub from a quill. Cursive script looked as though it were handwriting copied with some finer point, and a flowing pen.
Before papyrus and paper, readers expected inscriptions. They looked at letters that were carved in stone. Depending on the time of day, the light struck the stone at various angles; depending on the depth and the angle of the cut, the letter appeared as varying sizes. Even the shape of letters might vary with the time of day.
Some people think that ‘serif’ fonts, which have cross-parts and curves on letters and do not have letters which go just straight up and down, come from the days of painting prior to inscription, when the paint might run down at a corner. Whatever the irretrievable, historical truth, the abiding truth is this: so-called ‘colour contrast’, like sunlight on inscriptions, also interacts with font and point size.