Answer by
LeheckaG (1826)
Chemicals/Nitric-acid can cause brown-stains.
More-commonly,
"Brown/Lentigo-senilis/Liver/Old-Age/Solar-lentigo/Senile-freckle" spots
are caused by
chloasma/hyperpigmentation/melasma/solar-lentigenes,
from brown-Eumelanin & red-yellow-Pheomelanin,
produced-by melanocyte-cells/tyrosinase-enzyme,
usually triggered-by SunLight/UltraViolet-radiation-exposure.
Occasionally/sometimes, they are related to skin-cancer.
Skin regenerates-less Ages-40+.