Architectural work is seen for decades, mostly in daylight, so colour retention carries as much weight as film hardness.
A polyester or super-durable polyester system is specified for every exposed job; epoxy is an indoor material and chalks outdoors.
Tone consistency matters across large surfaces. Panels for a single facade are coated in the same batch wherever possible; RAL code and gloss level are logged for later additions.
Deep, saturated colours fade faster than greys, whites and blacks — a more stable tone is usually the safer choice for large sun-facing facades.