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Some garage door finishes can fade very quickly if they are in direct sunlight facing south or west.
Cheap paint will fade very fast and a high quality paint or laminate foil or printing finish is essential on any externally fitted door in the extreme climate conditions in the UK.
Door fading will depend on the UV stability of the paint used on the steel or aluminium door surface along with pretreatment processes too.
An example of high quality finishing is with Carteck steel side hinged and sectional doors as they use steel panels colour coated and tested to EN10169. The Trend colours used have a rating at Ruv3. EN10169 is a test using accelerated UV exposure to measure the fade and gloss retention over time. 4 ratings exist between Ruv1 and Ruv4 with Ruv4 being the highest rating for stability.
Garage doors can be finished in powdercoated, laminate foil and printed option generally for steel and then multiple options for timber doors.
The best advice is to get reassurance on the warranty for the paint finish but do some research where you can. Cheap paint finishes can fade within a year where high quality paint finishes can still look the same after 10 years if kept clean.