TIL, "In
software development, a greenfield project could be one of developing a system for a totally new environment, without concern for integrating with other systems, especially not
legacy systems. Such projects are deemed higher risk, as they are often for new infrastructure, new customers, and even new owners."
Source: Wikipedia
Well if you want to make new apps from scratch, just do some market research, get some ideas and start working? Are you looking for new ideas?
Anyway, just building an app doesn't guarantee income. Unless a sizable population downloads your app and you find a way to monetize you are not going to make money.