John, Adrian, can't tell a lie, it was me guv, only now I can't find the thread. It wasn't on my Comet thread.
The best bet for your Firefly, if you're at all interested in accuracy, is to paint it in Olive Drab. This is not however the conventional US Olive Drab. I'll explain.
From Apr 1944, the standard base colour for British AFVs in NW Europe was SCC15 (Standard Camoulflage Colour 15).
It was formulated to avoid the necessity of repainting US Lend-Lease equipment. Most US supplied equpments retained their US Olive Drab and UK built vehicles were painted in SCC15 with the result that the fleet had similar colours but not the same.
Where US vehicles were extensively modified in the UK, and Firefly is probably one, they were almost certainly repainted in SCC15.
So how to do SCC15? Dick Taylor's books are excellent. The other reference, which draws heavily on the source War Office orders, is Mike Starmer's monograph "British Army Colours & Disruptive Camoulflage in the UK, France and NW Europe 1936-45" (available direct from Mike). He provides accurate colour chips. If you use acrylics, the easiest approach is an equal mix of Vallejo 70888 Olive Grey + Vallejo 70924 Russian Uniform WW2. Alternatively, if you prefere Tamiya paint, the mix is: 5 parts XF81 to one part XF58 and one part XF71. Finally, if you use enamels, White Ensign Models do SCC15 but they never seem to have it in stock. You could of course use a car paint mix but the colour matches are difficult to establish with any precision.
Personally, I think given the effort that goes into our models, that taking trouble to get the paint scheme accurate is worth it. It's so easy to slap on any old car paint mix and end up with something that isn't very convincing.
Hope that helps.