Actually if you build yourself a current source as a project, you can see what colours red LEDs go above 20mA or so. It's quite interesting. Wear safety goggles though as the no brand chinese ones tend to explode.
I found out that a unlimited green LED turns Amber before the Raspberry PI it's connected to dies when it's cobbler is wired backwards. Thankfully it wasn't my PI, it was a work owned one. The LED didn't light when connected sensibly, so bring a little flippant I stuck it across the +- rail of the breadboard. It went bright amber, then dimmed and the PI went off with the LED. Neither worked again.
I never did check to see what I pulled the current through or into when the cobbler header was wired backwards. Something that didn't like killing an LED, I suspect.
On torturing LEDs, I think the Tenma PSU will do 10mA increments, actually think will let me set 1mA increments, but the spec says 10mA resolution (I believe). That should be enough to destroy an LED or two. Don't have safety glasses though, but I can use a plastic screen.