It seems the torpedo boats were repainted in a light grey by mid-war, and your boat. At Jutland, there are accounts of the German boats being much hard to spot than than the darker British destroyers, and there are a fair number of late war pictures showing light color boat. However, those were fleet boats, and the A-class boats were much smaller and operated with the Flanders flotillas in the Channel so it doesn't necessarily follow that they would be the same color, even in the same time period. I was curious about you question, and I found an interesting picture of the A-class boat A68 right on Wikipedia of all places. A68 would have been from the preceding class as A86 but was very similar as you can see. They were both war-built boats, A86 in 1917, I believe, A68 would not have been much earlier. As you can see A68 is in a very dark color (black?), much darker than the fleet boats, and the hull numbers are clearly visible. The deck looks like it is the same color. So, there is some evidence of a similar ship from a very similar period, with the dark paint and hull numbers. Gives you some options at least.
This is interesting: I just found another pic of A59, of the same class as A68, and another boat that was also given to Poland after the war and it, too, is in the dark grey (black?) paint with the hull numbers prominent. Here's the link:
http://www.weu1918-1939.pl/orp-slazak/ Maybe that's the way to go for these A-class boats?
(By they way, I have forgotten to sign in originally so this may show up duplicated as a guest post)