If the Aircraft is leased and not owned the return from lease conditions usually state that the Aircraft must be returned to lessor ALL white. So paint it into whatever livery (circa £20k for an ATR plus 3 days out of service plus a two ferries to/from paint shop) then paint it back to white at the end of lease (another £20k plus the 3 days and a couple of ferries again) and your looking at £45k plus a couple days out of action. Money that doesn't necessarily need spending in some Airlines eyes.