Red leds run on about 2 volts, white on about 3, and blue on about 3.3. On the side of batteries, they usually say their voltage. When you put batteries together, you just gotta make sure their voltages add up to less than what the led is supposed to run on. Blue would run fine on two AA batteries, although brand new alkaline batteries usually end up at about 1.8v each, so it might be pushing it. You just gotta find the right batteries, or better yet, use the diode test on a multimeter.