ABS is a copolymer made by polymerizing styrene, Acrylonitrile and butadiene with a uv Additive added during the Extrusion process.
UV stabilisers will reduce the degradation of the surface of the ABS. But will not stop it from changing colour.
Material with UV Stabilisers added, will ensure that the Mechanical properties of the ABS will be extended to a life as required. This however will not prevent the Butadiene Rubber in the composition of the ABS to Yellow.
ABS can be used for outdoor exposure but the yellowing problem is normal and cannot be prevented by using any U.V. Absorber.
In ABS UV stabilisers will only protect the Acrylic and Styrene part but will not protect the Butadiene parts and the butadiene is unstable.