What are common causes of a discoloured tooth (darkened) after root canal treatment?

It depends on what you mean by "last". If you mean how likely it is that the root canal treatment will have a successful long term outcome, then there are studies showing that root canal treatment in teeth with areas of bone loss around the root tips are about 70-80% successful over the long term. This is because teeth with this sort of radiographic appearance tend to have a much higher load of bacteria in the root canal spaces.

These are the materials that may be used for an upper premolar crown: Lithium disilicate A type of aesthetic and translucent porcelain that can be made with layers to mimic the appearance of a natural tooth. This material requires the greatest amount of thickness to withstand cracking and to give the best appearance. This material can also be used for partial crowns (onlays). Zirconia Another type of very dense and hard porcelain that can be used in thinner section without compromising strength. This material is also used for the blades of ceramic knives.