Color Oracle is a free, open-source color blindness simulator for Windows, Mac, and Linux that applies a full-screen filter over whatever you're designing in, in any application.
Because it filters the entire screen rather than working inside one app, it works with any design tool, not merely ones that ship their own color-blindness preview, and it simulates deuteranopia, protanopia, and tritanopia with a single keypress each.
Designers who want to check color-blind accessibility across an entire application or prototype, not merely a single exported image.
It's a downloadable desktop app, not a browser tool, so it needs installing once. After that, switching simulation modes is a single keyboard shortcut away.
