xctwdiff reads the message of a failed XCTAssertEqual check from stdin and runs wdiff and colordiff
on the two sections. It assumes you don't write messages in your equality checks.
It's a Xcode command line tool project, so build and archive and copy the binary out of the archive somewhere along your path, or alternatively add a hashbang line to the top and use it as a script.
It's written in Swift 4.1.