I bought a package of dwarf trackside signal lights (assembly required).  It came with red and green LEDs and resistors.  I assembled the dwarf unit and wired the LEDs so they both work at the same time.  I'm a dinosaur who just wants some color on my layout.  Anyhow, they light up without the resistor using a 3v battery (both at the same time).  So far so good.  So I add the resistor to the 'hot' side to be able to light them up off my power pack. Now only the red LED lights up when connected to the power pack using the resistor.  I'm stumped.  Any thoughts would be appreciated.