As requested, here's a youtube video Panamian Buses that shows some buses in Panama. Sorry about the music on it, but it does show a decent variety and there's plenty more videos on the Suggestions panel - or you can just google search "Panama buses". Some of the modifications are a question in taste, but it does make public transportation more colorful and fun. Enjoy!
*UPDATE* I found some more videos if you want to see more (with better music)
Night Time Video
I have a lot of family in Panama, and the buses are something I clearly remember even though I was about 11 years old when I was there last. I don't know all the dynamics, but I'm pretty sure all the buses are privately owned and don't operate on any type of mandated schedule - it's more like the bus operators are there to help move the community and get payed directly for their services. It's definitely a cultural attraction and as you can see, there is a sense of pride in their vehicles and there is usually no shortage of passengers. Could this be something to learn from... can artists make public transportation more attractive to people?