When they come in contact with CO2 they change colour to a yellowish brown.
BBC(May 31, 2016)
He extracts a yellowish brown lump the size of a golf ball and sniffs it.
Scientific American(Jun 16, 2015)
Mrs. Pontellier’s eyes were quick and bright; they were a yellowish brown, about the color of her hair.
Kate Chopin, The Awakening(1899)