Algodoo

Not sure if this works properly on other screen sizes but...

On scene start, stare at the white circle to the left. Tap D, then hold W. The white box will disappear to reveal a small circle that is either red or green. Without looking away from the white circle, determine the small circle's color. Once you have a guess, while still holding W, look at the small circle.

If my spacing is distant enough here, you shouldn't be able to reliably determine the color of the small circle. This is thanks to very few cones in your peripheral vision, the cells responsible for detecting color.

The reason you don't *feel* as if you're peripherally colorblind is because of your brain's ability to fill in the gaps. For example, there's a blind spot in both your eyes, the spot where the optic nerve attaches to the retina. I might make a scene demonstrating this blind spot too.

upd: darkened green