That's possible, Fivefootthree, and I had wondered myself. When the loco was painted grey at Whitehead some 15 years or so ago, it was correctly painted grey all over. Subsequently, on enquiring why the chimney and smaller kebox looked darker, I was told that since the Whitehead folk thought a grey chimney looked "odd"*, they had allowed it to get darker by not leaning it, whereas they DID clean the rest of the loco.
I would be disappointed to hear that it had actually been repainted black, as this is categorically incorrect. Irish preservation, unlike the U.K. or beyond, has an unfortunate history of livery inaccuracy, with virtually every preserved coach or wagon, across the RPSI, DCDR and Cultra, in wrong liveries. There are a number of reasons for that; as a one-time carriage restoration volunteer in the RPSI, I can confirm.
Comparing 186 with other locos would not be relevant; it's a different livery - indeed, the plain grey is the livery of the vast majority of steam engines. Between the mid 1910s and the early 1960s - half a century almost, grey locos on the island of Ireland outnumbered all the others by about 3 to 1.
186 should be all grey. So should 184 and 461, as these locos were this colour throughout their lives in their current state (though one photo strongly suggests 461 as one of the small number which got a coat of black in its final few years). In current form, 184/6 were entirely grey always.
Lovely model above, Eoin - grey possibly slightly light - however - while the grey used, like all livery colours, remained pretty standard, jhbSenior recalled seeing a J15 or possibly J18 (I can't remember which he told me), being shunted out of the paint shop at Inchicore in a lighter grey, same shade as wagons. Also, when brand new, the loco grey tended to look lighter though it didn't stay that way long! So your model is in "ex-works" condition.
Hope this is helpful.
Incidentally, Eoin, where did you get those buffer beam numerals? They are absolutely perfect!