Most cases come with orange and plain green, so I'd narrow that range down to 2v+/-0.1v Not to say you can't run a 3.7v blue, the source voltage is usually high enough. There's a resistor or some current-limit circuit already fitted on the mobo and I'll bet its set for under 10mA.
hrm, theres the nifty tool called a multimeter... lol...you're probaly right about it being low current, as most LEDs that come in generic crap cases are very dim.
