Hi Kirk,
Haven't we just met elsewhere ??
Anyway,
RD240 is perf 11
RD257 is perf 10.
As to the colour, I have checked some yellow-green stamps and I would say this was green, but I would not swear to it. It could well be yellow green, but on my screen it looks more green !! [That rhymes

]
If it is green, then it could be R240a.
Better check the perfs first !!
Londonbus1