I'd never trust a developer that's only worked on green field projects, they're oblivious to the mess they leave because they aren't there long enough to feel the pain of their design decisions. So you've got one up on a lot of people there.

Aside from your own projects, look for opportunities for other projects at work where you can start with a fresh technology stack. Some of these projects might be taking over the non-core functions of the main app. For instance, chances are a lot of the UI is sub-optimal (generic crud based) for some specific users. You might be able to create a slicker interface that makes it easier for them to do specific tasks that feed that data into the main database.