There's no evidence whatsoever to suggest the cowl was yellow. Alex Imrie did an interview with Karl Timm, who was Voss's mechanic, in 1961 and showed him a few photos of F.I 102/17;
"Herr TIMM states that this aircraft was colored grey, the foward part of the fuselage and the engine cowling were a very dark grey, to use TIMM"S own words an "earth grey", while the shade used aft of the cockpit and on the tail unit was "silver grey", all under surfaces were coloured light sky blue, and the top surfaces of the main planes were streaked in dark and light areas with various shades of grey. Herr TIMM has no recollection of doing any additional painting on this machine, he maintains that it was flown as delivered.".
On top of that, the Jasta 10 colours at the time were yellow noses and tail. However, in all of the photos of 102/17 the rudder is clearly white. If the cowl was painted yellow, as per Jasta colours, then why wasn't the rudder and tailplane also painted yellow? If there was time to paint the cowl, then there was time to paint the rudder.
Also, the three F.Is were effectively prototypes - only two were sent to the front to be evaluated by Voss and von Richthofen. There was never the intention that they would remain there, so there would seem little point in painting them in Jasta colours as they would be returned back to Fokker after the short evaluation. As it turned out, both had been shot down within a month of arriving at the front.
Finally, the 56 Squadron Combat Reports from Voss's final flight make no mention of a yellow cowling, but do mention that the Fokker was 'silvery grey'.