I can't tell you for sure, but my assumption is that assigning colour-block avatars to new members has been disabled for a while. A few other features have also been temporarily shut down to help the site run more smoothly while the server is either optimized or expanded. 

And don't even ask about my avatar.

EDIT: Oh, and you won't be a blank slate forever. A properly functioning avatar system that allows users to upload custom avatars is in the pipeline.