For somebody who’s not colourblind, it can be a bit hard to understand how the world looks if you are.
However, I’ve just discovered there are now sites that can help.
For instance, here is the same photo of Anna processed in three different ways by Colour Blindness Simulator:
From left to right, that’s the original, then the way it’s seen by people with protanopia, then deuteranopia, and finally tritanopia.
This is of course really useful if you’re designing user interfaces so that you don’t unintentionally create designs that are confusing or unreadable to colourblind people.