Why is the WhatsApp button's label dark?

Because white on WhatsApp's bright green cannot be read: it measured 1.98:1, where the accessibility standard (WCAG AA) asks for 4.5:1. From 1.4.32 each button's label colour is worked out from the network's colour, so WhatsApp, Telegram, Reddit and Bluesky have dark labels, and Facebook and Mastodon keep white ones on a very slightly deeper blue and purple. Outline, Soft and Minimal use a deeper shade of the colour where the brand colour is too pale to read on the page. The pinned bar also stays light now, rather than turning dark with the visitor's phone, where those styles put black marks on a black bar.