Dubh Artach lighthouse lies about 15 nautical miles west of Colonsay and was built between 1865 and 1872 by the "lighthouse Stevensons" for the Northern Lighthouse Board (NLB). NLB account of the constructiuon is at
Link.
Also, a brief account of his visit by Robert Louis Stevenson is at
Link - "An ugly reef is this of the Dhu Heartach.... No other life was there but that of sea-birds, and of the sea itself, that here ran like a mill-race, and growled about the outer reef for ever, and ever and again, in the calmest weather, roared and spouted on the rock itself."