Matt;
Fire bottles came in a wide range of colors, although the red, blue, and yellow ones were much more common earlier in the war. The Germans used a small number of standard commercial models made by the Tetra company - typically these came in industrial colors, including semi-gloss dark gray, green, red, blue, yellow, with various logo and instructional decals pretty prominent under the handles. Later in the war it appears evident that many tank crew would paint them along with camo coats, and many pictures of historic relics show that the original shiny colors were often over-painted with dunkelgelb. For all I know, rear-area shops may have repainted fire bottles in DG on a regular basis? Most bottles appeared to have their labels and decals pretty visible and intact - thus those Tetra decals we see.
For a 1942 - '43 DAK tank, probably a chipped red one would be OK, as tanks were hard to hide in any case out in the desert. Late-war tanks may more often have carried repainted bottles - not that a small red bottle would make a 40-ton tank suddenly substantially more visible to a roving JABO!
Bob