In AE CS5.5 how do I keep my composition background color from rendering? My project is composed of titles and shapes and I want the background to be transparent.
First you should study up on video formats and compression so you know what should be trying to do. You can find starting information by typing "rendering" in the search help field at the top right corner of AE and browse through the community resources.
Second, if you just want to dive in there are a few things you should know. Media players like the Windows Media Player or the QuickTime player do not display transparency. Transparent backgrounds will always be black. Most editing apps like Premiere Pro, Vegas, Final Cut and even iMovie and Windows Movie Maker will recognize transparency so you can add your titles in any of those apps and see through the background. The best color mode (research that) is Straight Alpha. The After Effects Render Cue has an Output module and one of the Presets is Lossless with Alpha. This will give you a usable file. Do not be alarmed by the huge file size. If you want to be a pro you'll open up that Lossless With Alpha preset and look for the line that says Pre-multiplied and change the mode to Straight Alpha. You'll get a better result.
So there you go. If you intend to do very much video work I strongly suggest that you learn all you can about formats, channels, compression, color depth and video standards for frame rates and frame size. Until you do make sure that your Composition Settings and your Render settings do not say custom because that means you've changed something and it's impossible to tell what you are going to end up with if you are just guessing. Most of the time guessing will be you unexpected results.