I own a Roku Premiere 4K (also a a Roku Express) and there's no Acessibility on settings menu, but there's a subtitle option that let me adjust the color.
The problem is that the "white" option is just a gray (not even close to real white) and all the apps that use roku subtitle settings stick to that color.
Netflix, that manages its own subs, shows a bright white, perfectly readable, making me believe it's not a hardware limitation, but a software one.
It's really frustrating trying to use Plex on Roku (this one uses roku subtitle settings exclusively) given the poorly visible subtitles.