What causes the hands to change color after using the pressure gauge for a period of time？

What causes the hands to change color after using the pressure gauge for a period of time?

Except for special purpose and required pressure gauges such as marine pressure gauges, the pointers of general pressure gauges are black.

After some pressure gauges have been used for a period of time, users find that the color of the pressure gauge pointer changes from black to approximately light red or off-white. Especially for pressure gauges used in the open air, the red wind blows the sun more clearly. What is this? the reason?

There are generally two methods for surface treatment of pressure gauge pointers, one is oxidation treatment; the other is spray paint (plastic spray) treatment. The oxidized pressure gauge pointer has a delicate surface and beautiful luster, but the disadvantage is that after using the pressure gauge for a period of time, especially after the pressure gauge is used in the open air, the pressure gauge is liable to fade; and the painted pressure gauge pointer, Although the appearance is not as delicate and beautiful as the oxidized pressure gauge pointer, the pointer of the pressure gauge rarely fades after long-term use.