Sorry, I missed your question earlier. From what I can tell there is a very slight fading of the black finish but the sign remains dark gray with a pale gray or off white image. Quite legible and looking like they will hold up for years.

I have experience only with black in an outdoor application and that's what MOBOT Garden uses as well.

I did do a job for a suburb of St. Louis recently using gold Alumamark with a UV spray on after engraving. These signs were to be placed in gardens. I expect these to hold up for three or four years but told the buyer I would replace them if they faded.