I find that Miniatronics yellow 1.5mm LED packs fullfill most of my needs, but occasionally some of these colored 'accent' lights really come in handy. You can buy just red or green, or go for all 3 colors with their multi-pack. I've used a green LED to illuminate a sign on the side of a local pub for example. These have the long leads that make soldering wires easier. They also come with the resistors you need to get them to work with your set's power pack. They are small enough that they can be hidden as needed, or even shown with the rounded 'bulb' exposed and the rest painted black.