We're implementing different color themes across our apps.
All our basic themes are WCAG level AA compliant, but now it's high contrast theme's turn. It's going to be a theme that features maximum contrast for - hopefully - the best accessibility (we're aiming at level AAA and beyond).
Now, intuitively, it seems that the background of
#000000 (black) and foreground (text, icons and other data) of
#ffffff (white) is the way to go, since this combo provides maximum contrast of 21.
What I keep seeing in different accessible usecases, is high contrast themes using yellow
#ffff00 for foreground, which provides the contrast ratio of 19.56 (which is still great, but less than white). Among yellow, bright green and bright blue are used too, but i feel like yellow is more prevalent.
The closest to a standard or guidelines I've seen for that are Microsoft's XAML theme resources reference for Windows 8, that also feature those colors, but makes them more contextual.
Government portals of Latvia make use of multiple high contrast themes, among which there's black-and-white and multiple black-and-yellow themes. And some just feature black-and-yellow for high contrast mode.
The question is, what's so special about yellow? Why do they go for bright yellow instead of white? Does it have any significant upsides for certain types of colorblindness? Do we have any studies or research about that?