Test your judgement
Which “Get started” button can the most people read?
Same button, same colour family, different contrast. Pick the one you think the most people can read - including in sunlight or with low vision.
Version B works better here. Version A's pale-on-pale label may look tasteful on a bright designer's monitor, but it falls apart in sunlight, on a cheap screen, or for anyone with reduced vision - the text and its background are simply too close in tone. Version B holds up because the label and the fill are far enough apart to stay legible everywhere.
This is colour contrast: WCAG asks for a ratio of at least 4.5:1 for normal text (3:1 for large). It is not a nicety bolted on at the end - if people cannot read the button, every other decision about it is wasted.
The honest caveat: contrast is a floor, not the whole of legibility. Size, weight, the font itself and the busyness behind the text all matter too - passing the ratio is the start of a readable button, not the finish.
See all the checks · warm up in the glossary or test the terms on the quiz.