You could look at triggering different lights with amplitude, like a VU meter. This is a cheap 5 LED digital VU meter ->
I've used it before to drive some LEDs on a kinda birthday card thing, I plugged the card into my Ipod output, and the 5 LED flowers on the card flashed with the music!
You could hook the output pins to some transistors which could then drive your colored light bulbs, it would be much cheaper and easier than using a micro controller. This would give you a regular audio level meter much like the one on a mixing desk, you could however change the order and color of the lights to mix it up a bit, so it's more of a light show. The lights also fade in with the increase of amplitude rather than just flashing on at a set value, so it would look cool.
Otherwise you would have to take a look at frequency analysis, whereby the lights could be triggered by the audio level at different frequency bands (one per light). This system would closest resemble a disco light that flashes with the different instruments in an audio track, but it's much harder to implement when compared to amplitude analysis. I'm sure a modern MCU would do the job with ease tho, with only 5 lights the resolution on the analogue inputs would not be a huge issue. If you could find some code to steal this option wouldn't be that hairy, if you're familiar with micro controllers.