Question: How do you know if you're seeing the same colour as someone else? Like I'd see a blue pencil, but in another person's eyes they would see it as their 'blue', but it looks like my pink?

The easiest way to test out if you see colour differently is to ask. People can see shades of the same colour differently (eg. I may think something looks more blue, while you might think it looks more purple). These differences can be because from where I’m seeing something, the light could be affecting it differently or that because there are some variations in how the receptors in everyone’e eyes detect colour and then how the brain interprets those signals. eg. someone with more receptors in their eyes for blue will see more blue in an object than say, someone with more green receptors.

But if someone else sees a colour completely different – like you said, blue looks pink to someone else, this could be due to a problem with those receptors. The most common is colour blindness, where certain receptors in the eyes, usually the ones that detect red or green don’t work properly, and because of this instead of seeing red or green the person will see a different colour or no colour at all. There are some simple tests you do can to see if you are colourblind, based on seeing coloured numbers amongst dots (it’s known as the Ishihara test). You can do the test here: http://www.toledo-bend.com/colorblind/Ishihara.asp