Colour blindness

I myself am protoanomalous, meaning that I don't perceive red as a bright colour at all. Instead my world has two kinds of "green". Standard RGB based displays cannot reproduce the world I see. It's as hard to explain as being able to see octarine, the colour of magic. I once almost trod on a fox (reportedly a kind of orange) lying in grass (famously green). Thankfully the fox was more aware of me than I was of it. All I can offer is that foxes are well camouflaged if you can't see red!

There are sites out there on the web that claim to be able to simulate how someone with one of the various colour deficiencies would see your image (or in the case of a Ren'Py game, a screenshot). I honestly can't tell you if they work! You might suspect that I shouldn't be able to tell the difference between the before and after, but I can.

Similarly there are sites that claim to measure contrast between user interface elements. These don't take into account colour deficiencies as they make an assumption that green is the brightest colour, then red, then blue. Mine is green, blue, red.

I've played triple-A games with supposed support for colour blind modes, and each one has failed horribly. What hope is there then for the lone developer or small team? I'm of the opinion that the only real test is the mk.1 "defective" eyeball, and you'll need access to the different kinds of "defective" eyeball if you want to include colour in a game and keep accessibility.

Challenges

These are mine...

Health bars, near death indications

These are a particular bugbear. If you use red to highlight a problem I can almost guarantee I won't see it. I get it, red is danger, green is safe. But there's a reason warning signs are often yellow/black.

Colour matching games

Yes, but no. I'm going to be much slower at these, even if you add different shapes or designs. If it's against the clock the simplest solution is to give me more time.

Colour pickers

Thankfully these are rare in visual novels. Being curious about how others see the world I've actually got a good working (if theoretical) knowledge about how colours are supposed to work. That isn't going to be the case for everyone with a colour deficiency. But let me name the colour I pick, or choose textually.
See XKCD color survey results