super-bright LEDs are bright enough to light up a car's cabin at night. you'd only need one or two for each footwell. it's really easy to wire them up, you just have to make sure there is a resistor between the LED and the power, or they'll burn out instantly. as for colors, you'll most likely be limited to blue or aqua.

edit: and you'll have to figure out a way to mount them. that'll be the hardest part, in my opinion

i bought a set of cold cathodes for my car. they give off a ton of light, but they're a bitch to mount and the "red" color i got is actually more like pink. i only thought about using LEDs after i bought the cold cathodes.