Full description and workaround here: https://binkley.blogspot.com/2017/04/maven-color-logging-on-cygwin.html
In short: the JANSI library used in Maven 3.5.0 does not detect Cygwin, and falls back to no color. There is a workaround (Zart Colwing): Add -Djansi.passthrough=true to MAVEN_OPTS.
As the mvn script understands Cygwin, it would be helpful to add the passthrough flag automatically. (Of course, there may need to be some smarts about batch and logging modes, and checking if stdout is a console vs pipeline.)
Apologies: I'm unsure how to issue a PR for the maven repo.