The WW1 St Chamond and A7V tanks were fairly colourful but not that colourful. As far as is known the French Schneider and St Chamond tanks were factory finished in "horizon blue" - a fairly dark blue-grey colour. The crews applied various camo schemes with ochre, green and dark brown sometimes with black boundaries between the camo colours.
I'd suggest getting hold of a copy of the AJ Press book on the Schneider and St Chamond for a realistic take on the colours of these tanks.
The A7V colours are less well understood although they've been a subject of research for many years. The base colour seems to have been "feldgrau" - a mid-grey colour with a green or brown tinge. The camo colours seem to have come from captured Belgian railway paints - dark green, red-brown and a pale cream colour. The famous book on the A7V by Hundleby and Strasheim tries to identify the colour schemes used on the various A7Vs - this book is long out of print and very expensive these days. The colour schemes were different for each of the 20 A7Vs fielded and changed with time in service.
The current colours on #506 Mephisto aren't a good guide to A7V colours - it was repainted in the 1990s and it's too green. Best guess for Mephisto was red-brown areas over feldgrau with no green.
Regards,
Charlie
