Public Domain
What do you see when you look at the blob above? I see just that: a blob. Some colorblind people will see, apparently, the number 21. Color-sighted folks will see a 74.
This image comes from the Ishihara test, one of the oldest methods for determining whether or not a person is colorblind. Usually, a colorblind person has a reasonably good chance of figuring it out for themselves. Speaking for myself, I found out as a toddler when my dad started pointing out trees turning red, yellow, and orange for the fall and I had no idea what he was talking about.
As part of a look into the science and experience of colorblindness, I had the chance to, er, test out many of the available online tests for colorblindness.
The best one I found comes from a company called EnChroma. It's part of their marketing for very expensive sunglasses designed to make colors easier to distinguish for colorblind people. (I'm also currently reviewing a pair.)
Here's my result:
Enchroma
In other words, I have a fairly normal, boring form of the defect.
Here's a link to the test, which takes just a few minutes to complete.