The cabs, yes indeed - but the side tanks?
Either way, an ingenious conversion!
The livery for the GSWR was lined dark olive green as shown above until about 1902, after which black lined in red and white became the norm. The all-over dark grey first appeared about 1915, possibly following the extensive re-boilering programme at that time.
During the currency of the dark green, lining was pale green, red and black up to about 1880 or so, with same green lined in black and cream later.
Frames were mid-brown during the green livery.