Water-based acrylic paint manufactured by Games Workshop.
Citadel "Layer" paints have a lower pigment density than "Base" paints. 
They are designed to be applied in multiple thin coats over a base color to build up rich saturation, brightness, and smooth transitions.
It is ideally suited for creating crisp, eye-catching highlights on edges and raised surfaces to make miniatures stand out on the tabletop.
Formulated to work seamlessly over other Citadel paints without requiring complex color mixing. 
It washes out easily with water and can be thinned using water or acrylic medium.