G'day Troy - a fair question
I am using two primary references for this scheme, the first is the large photo in the "Fokker D.VII in Action" publication, as well as the instruction book from the Americal-Gryphon Jasta 4 decal sheet which includes this scheme. The instructions have a particularly lengthy blurb about this aircraft and mention that it most likely served in Jasta 6 (where it picked up the Jasta 6 white and black tail stripes) before it was transferred to Jasta 4. The instruction sheet goes on to say that it is known that Jast 4 used a home-made mix of "grey-green" to overpaint fuselage crosses, and as such it is likely that this colour was used to overpaint the tail plane; the paint being thin enough to still clearly discern the black and white stripes (which are the stripes that can be clearly seen in the photograph).

This goes in line with other info I have on Jasta 4 aircraft which shows the only unit markings were black cowls and struts - overpainting the tailplane in this manner would bring the aircraft markings roughly back in line with the rest of the Jasta.
Cheers,
BC