Airlines that adopted another's livery
Inspired by the Viscount thread running at the moment Channel Aiways adopted Continental's then colour scheme when they purchased their Viscount 810 fleet, and went on to use it on their Trident and 1-11 fleets.
More recently Jet2 has adopted the Allegiant scheme for their 'Holidays' aircraft.
Are there historical examples? I think the yellow livery that Northeast (UK) used before they were absorbed into British Airways was at least loosely based on their American namesake. Apart from that I can't recall any other instances.