jst becoz of E=hf & E=Vq :D
u need to excite electrons of atoms ne
and f is high in blue than red...
at 20mA 6-3.7=0.02*R
:D ;ask if u need more help :D
chk at data sheets....i cant remeber......
then use Si diode to drop voltage (0.6V per one) or use appropriate resistor...it's easier than using regulators :D