Dark shells differ from white shells in that dark shells are leased to tenants without any interior improvements, such as heating, lighting, interior walls, plumbing, or air conditioning. White shells, also referred to as warm shells, vanilla shells, or white boxes, typically have heating and A/C, plumbing, restrooms, and lighting, but may have other parts of the interior that are unfinished. Source
In many cases, a building may start out as a cold shell, but with the agreement that the landlord will make the necessary improvements to convert the building to a warm shell once the lease agreement is fully signed. Sometimes, a tenant improvement (TI) allowance will be offered in order to defer some of the costs that a tenant will incur by improving the building themselves. Source