Nowadays most projects are now described as brownfield or greenfield. To define it without going into too much detail;

Greenfield projects are projects where a project is made from scratch in a new environment without any restrictions. In other words, it is not an upgrade or rearrangement of any old system, but the establishment of a new system. An example project would be to implement SAP in a company that uses an ERP other than SAP. These projects are also called blue sky projects.

To list the advantages:

• No process design constraints
• No obligation to comply with existing systems and infrastructures
• Latest applicable software versions and methods applicable

Disadvantages:

• There are endless options because there are no constraints
• Rebuilding all software from scratch is risky and demanding
• Major decisions may need to be made to move the project forward

Brownfield projects, on the other hand, are the improvement of an existing software or projects made on existing software. To give an example, projects for upgrading your existing software, adding new modules or processes.

Advantages of Brownfield projects:

• The scope and constraints of the project to be made are predetermined
• An opportunity to improve existing software

Disadvantages:

• Existing software and technology must be well known, otherwise wasted effort.
• The fact that the existing codes are not of the desired quality also creates a risk for the new project.
• Existing software may also need to be re-examined within the new project.
• The current process and software constraints need to be analyzed correctly in the project.

Of course, there is a hybrid model of contrasting concepts such as greenfield and brownfield, and such projects are called Bluefield projects. In a project, if some parts are designed from the existing system and some parts are designed from scratch, they are called bluefield.

Further reading