is a specific kind of: achromatic color, achromatic colour.

is a broader term that covers: ash gray, ash grey, silver, silver gray, silver grey | charcoal, charcoal gray, charcoal grey, oxford gray, oxford grey | dapple-gray, dapple-grey, dappled-gray, dappled-grey | iron-gray, iron-grey | tattletale gray, tattletale grey | Davy's gray, Davy's grey, iron blue, steel gray, steel grey.

derivation: gray, grey | gray, grey | gray, grayish, grey, greyish.