Why is the color arrangement of a rainbow permanent?

It is because of the different wavelengths of the different colours. White light is made up of a more or less continuous spectrum of light waves of different wavelengths and, in a prism (or a raindrop, which acts like a prism), these are bent or refracted at different angles, depending on the wavelength. There are many, many pages on the Web which describe why a rainbow is the way it is and how it works. Here is just one: http://science.howstuffworks.com/nature/climate-weather/storms/rainbow.htm I hope it helps! (It certainly helped me, because I used MyLot Search to find the page for you and earned an instant 19 cents reward!)

The color order will always be the same but there could theoretically be slight changes due to impurities in the water droplets that help to form the rainbow. However, you would not be able to notice this with the naked eye. The most interesting things to note are: You can only see the rainbow from one side. You can't be at the end of the rainbow (you wouldn't see it!) as mentioned in a number of songs.

because the water particles break up the light into its bandwidths, red being longest and violet the shortest

Well each color of light has a specific specific wavelength. This property affects how much the light is refracted by the water droplets -- so one color (blue i think) is always refracted by the greatest amount because it has the high wavelength. So the colors have fixed positions on the rainbow we see determined by how much they are refracted.