I think that LED's operate at the wave length of 550nm to about 700nm but I am not to possitive on that but it sounds about right to me!!!

Just go to Radio Shack buy a pack of blue LEDs I think they are 2 for $2.99 use 120o resisters color code (red red brown) although (red red black) works as well buy pretty much any volt as long as it not to high adapter and away you go. Or even better just buy a 1.5 volt adapter and thats what the LEDs run on.
Just my two bits
Cheers
Tyler