Yes. Originally probably painted cream upper and ceiling, mid brown lower. Later, very pale grey throughout. I may have interior photos somewhere but if you look at the DCDR website some of their photos may show it.
As far as I know, the G611s were always the very pale grey inside.
In Arigna Road times (I'm assuming that Arigna Road closed in 1957!), a G601 class was silver. So, the first thing is serious weathering! The silver was on the roof and chassis also, relieved only by full red buffer beams. Lettering was of the larger style, and in pale green (not the black seen on Silverfox models) with no "snail". No "G" ever carried a "snail" in any livery.
In reality, the silver will be pretty dirty, especially round the wheels and roof. These were still steam days, so not just brake dust (as today) but also soot and smoke had an effect on the weathering of locos and rolling stock.
By the way, at Downpatrick, the liveries of G613 and G617 are correct. That on G611 is not for a number of reasons.