I am making a table light with a 3v 5mm multi color changing led, an on/off switch and a 18650.
What kind of resistor do I need to put inbetween the led and the 18650, for it to have a long life?
The color changing led is a simple one that you get with solar garden lights that you can stick in the ground.
I only have one and don’t want to blow it.
Thanks
Those RGB LEDs usually have about a 3v forward voltage, and can take 20mA of current. So, you need to drop 1.2v from a fully-charged cell. 60 ohms should do it, or round it up to a 68 ohm resistor.
Most of the time the battery voltage will average about 3.7v, so you’ll actually be driving the LED at about 10mA. That will give you a couple of weeks of run-time. Maybe more, but the light will get pretty dim near the end.
That did it! thank you!!! 
Thank you CRX I saw some formula’s pass by but I didn’t knew how to find out the I or mA for my led.