Accessible software design doesn’t happen on accident. If I design based on my own instincts, the software will work great for someone exactly like myself — with all the same abilities and preferences. I can see the difference between red and green, so I wouldn’t think twice about putting those colors next to each other in a layout. It’s only when someone with colorblindness tries to use the site that it doesn’t work at all.
All good design starts with thinking about the audience, and accessible software design is no different. If, instead of assuming everyone would see it like me, I took the time to run my layout through color checkers for different types of colorblindness, we’d skip the problem above and anyone would be able to use the site.