Forward current for assorted LEDs
Start below 1 mA and observe the light output of the LED. Increase current until light is bright enough for your application, 3-5 mA typical. Non-illumination grade devices won't benefit from currents above ~10 mA so you shouldn't worry about overloading. Another good indicator is the temperature of the leads, when you can feel warmth with your fingers it's time to stop.

Be careful with violet emitters, they could be UV ones and emit just a small amount of visible light. Same for clear which conduct but not emit any light, that's likely IR ones. Old IR may have opaque germanium lens - don't put them in your mouth. Other than that, LEDs are safe to experiment with and quite inexpensive and if you're out of them just salvage some form old electronics.

Take a look at this too -> http://youtu.be/GAriT4B-gkA