In the Upstream Offshore Industry we consider Greenfield Construction to be the construction, installation and hook-up of a totally new facility. A facility that has never been in production, where there are no simulatinious operations such drilling or production in progress at the same time as construction activities.
Brownfield on the other hand is the addition of new facilities and equipment on an operating, producing platform. A Brownfield project usually means you are doing "Hot" work on a platform that is in production and may have drilling operations in progress.
Brownfield work can't be done in a "closed fence" environment so therefore is inhirently more dangerious and demands that you mitigate the risk.