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Starch.
Rusting
Mold
Over watering could definitley turn the tree brown. Back off a bit and the tree should recover.
Gravitation.
There are two evergreens that turn golden brown and then drop all of their needles in the fall. They are the bald cypress and larch trees. They are only found growing in the state of Iowa. There are other evergreens that are actually yellow in color all year round including the gold yew, and yellow ribbon arborvitae.
Brake dust!
Bugs are wiping their asses on them
Alkaptonuria is an inherited condition that causes urine to turn black when exposed to air.
Too much iron in the water
It is the high temp and the high return Pressure inside the injector that causes the fuel To turn black
Smoking turns the teeth a yellow-black color because of the tar that is in it.