These are part of the Productivity Power Tools for VS2013 (see here). Specifically they are part of the Structure Visualizer feature.
There does seem to be some meaning in the colours. I haven't been able to find a definitive list of colours, but I've noticed the following:
- Grey for general blocks, like namespaces, try / catch blocks and using blocks. The grey for namespaces seems to be a bit darker than other block types.
- Teal for classes
- Blue for methods
- Green for conditional blocks (if / then, switch)
- Purple for looping blocks (for, foreach)
You can turn the lines on and off in the Productivity Power Tools section of the Options dialog.