> accessibility is just usability. all it is is making sure your product is as usable by as many people as possible.
Not always. There are things that increase usability for fully abled people but decrease it for disabled people. Color-coding is a common one. Another is using smaller font size to make more info visible at once, or advanced touch gestures that can be accidentally triggered by shaky hands.
Then there are accessibility features that actively harm fully abled users, like sticky keys or screen readers that automatically follow focused UI elements.
Color coding doesn't decrease usability for disabled people (and I say that as a screen reader user). You just need to ensure that color is not the only way the relevant information is conveyed.
But you can — or should be able to — independently toggle all of those features. For example, the app should work in high-contrast mode with disabled color coding, it should work with touch gestures disabled, etc.
A good common framework helps here. Then, it's up to each user to set their settings properly.
Colors should be an OS feature. On iOS and macOS you can remap colors globally and it works in all native apps. There are presets fire the common cases: red/green issues, color blindness etc and you can make your own. This has its limitations though eg if you are not able to distinguish at least 5 or so colors… any colors.
That's why you don't just use colour for identification. E.g. include a warning icon next to a button/element in addition to using a warning colour.