What number do you see in the above image? You should have seen 57. If you see 35, or no number, you likely have colour blindness.

About 8% of Australian males and 0.4% of females are colour blind*. There are various kinds of colour blindness. You’ve probably heard of red-green colour blindness and know that’s why traffic lights are split out separately. Some people see colours differently than they appear to those with ‘normal’ sight, some only see varying shades of grey. Neil Harbisson has the rarest form of colour blindness, achromatopsia, and only sees the world in shades of grey:

I’ve never seen color, and I don’t know what color looks like, because I come from a grayscale world. To me, the sky is always gray, flowers are always gray, and television is still in black and white.

— Neil Harbisson, I listen to color TED Talk

Like Neil, many colour blind people memorise or pick up colour-based facts, but can’t actually see the colours:

Roses are red

The TARDIS is blue

Grass is green

Oranges are… well, orange.

While he knows these facts, my partner can’t tell things like the colour of my hair, or eyes. He can’t explain what colour something is, or describe what colour he is seeing.