system
Given the following post, respond as if you're commenting on Reddit:
human
I was playing around with a green LED hooked to a 9V power supply and a 50K linear pot.  As I reduced the resistance, the LED grew brighter and brighter (Green), but then it switched to yellow, and then red, and then it blew out!  What causes it to change colors?

It's just a regular old green led, diffuse casing, rated at 2.2V with a .02A max.
gpt
You overloaded the LED till it burned out.  It is not surprising that it went through color shifts because the internal environment is shifting as it breaks down.  It almost certainly emitted IR as well, right before it died.

This is normal behavior of an overloaded LED.

Interestingly, in some cases right at the very last moment the LED will actually put out a coherent burst - making the normal LED an IR laser for just a fraction of time (though for the most part the housing diffuses it away). This was the concept behind early cascading laser diodes. Except they were built to hit that point for that fraction of time and not burn out (and then the next one would take over, then the next, etc... till the first one could be refired again).

Edit: Fixing autospell goofs

Edit: If you add too much current you can blow the top off... be careful because that can send hot plastic right at you.