Glossary
Greenfield means building on an empty site. Brownfield means building while an existing system keeps running around you.
The difference is not technology, it is constraints. On a greenfield you decide freely. On a brownfield you decide against existing data models, live contracts and users who did not order a migration.
Almost every commercially interesting project is brownfield. Greenfield mostly exists at the start of a product or in a clearly bounded niche beside it.