It’s not black, as such; that much looks clear. If not black, it has to be green, as it’s obviously not silver either.
I have seen a picture of (either) an A or C so utterly filthy that you couldn’t make out the white bit at the top at all, but if this engine was (a) black, and (b) as exceptionally dirty as that, you wouldn’t be able to see such a clean number on the front. So we can rule that out.
A very small number of A & C classes were painted in the older dark green around 1960-2. While obsolete on diesel locos and passenger stock since 1955, it was still used on buses and lorries.
i think there were two or three “A”s and at least one “C” (231).
Some had lining, some not, same as the normal lighter green. There appears to be no rhyme nor reason behind this variation, same as dayglo patches on the fronts of 071s & 141s etc in the 1990s.
As for numerals on the side, and the metal snails that “A”’s generally had, these simply haven’t been added.
The loco could be the normal lighter green - variations in lighting on the day and the type of film can make blues and greens appear lighter or darker. I’m inclined to suspect this might be the case, however it’s equally possible it’s the darker “bus” green.
Cork and Limerick painted a number of secondary-use coaches in the plain darker green in late days.
Finally, re the snail. This lasted about a year after the black & tan livery came in. Same on the road - first repaints of buses into either red and cream or navy and cream have snails.
But no railway vehicle ever carried it in even the earliest BnT times.
The BnT livery on the railway first appeared in 1962, but was very rare until well into 1863. The “roundel” first appeared in 1963.