No, it is still not possible in Photos for Mac to compare two Photos side-by-side.
In the previous versions of Photos I used the work-around to add the two photos to a Calendar page or a book page, but the options to create an Apple-book has been removed in Photos 4.0.
I am using PowerPhotos as a second browser for Photos. This way I can view one photo in Photos and another one on PowerPhotos, but this does not suffice to compare the colors correctly.
You would have to export both photos from Photos and open them in Preview in different windows, very inconvenient.