I have seen green, black and grey on 105E engines and boxes, I reckon it could have something to do with where it was built, and maybe when. The grey is a Dove Grey used on Fergie tractors. To be honest I would paint the block either green or grey, would give you half a chance finding a leak.
Blue was Fords exchange engines, seen a few of those and my Dad had one in his Corsair. I’ve had several original Anglia cars/vans but there doesn’t appear to be any rhyme or reason to what colour came from where. Dark green or black are the usual engine block/head colours. Never seen an original car with grey. Come across several that have had Uniparts Gold Seal exchange engines, painted gold obviously. Once had a NOS complete crate 997 cc engine and that was all black. Nearly all gearboxes seem to be the dark green though.