Eclipse displaying one of the source folder (src/test) in different color compared to source folder - src/main?

This feature was introduced in Eclipse 4.8 to indicate that a source folder contains test code.

See New and Noteworthy, the section "Test sources" describes it in more detail.

I'll quote the relevant part here:

Test source folders and dependencies are shown with a darker icon in the build path settings, the package explorer and other locations. This can be disabled in Preferences > Java > Appearance