I'm not talking about the onboard LED at the Arduino board here. What those LED's mean, is the LED's we usually plug at the breadboard.
I plugged in a yellow LED and a green LED onto a breadboard. I wired it up, and noticed something. Green LED lights up at its full brightness, while yellow LED didn't emit any light. Tried in another yellow LED, and it seems like it's smoking, which means it's burned.
I'm not using a 220 Ohm resistor or any kind of resistor that time. I also have no idea why the green LED didn't burn itself, when you plug it directly on 5V and GND. I'm expecting that the green LED should also be burned, because I think, that's the way how LED's work.
When I plugged the green LED into a 220 Ohm resistor, it did as expected. It lowered its brightness, of course, because the resistor is resisting some of the electricity to pass on the LED.