If an insect which is already well-adapted to its environment is born with a mutation that affects its color, for the environment it is already in, it may be a disadvantage. However, nature is not static--on the contrary, it is ever-changing. Whatever the color change is could present an advantage, a disadvantage, or have no effect at all. However, if the environment should change due to pollution, deforestation, or any other phenomena, this mutation could prove valuable for survival. Thus, a small number of individuals could be "fit" in the new environment, and might live and reproduce--possibly passing the mutation to more individuals. Or, the insect with the mutation may die out as it may be seen easily by predators. The fact that DNA can change is what leads to evolution and changes over time.