If you are not dropping anything on your nails and they are turning into brown then it must be because of the picking you might have done at only one place and this may have ultimately resulted in a change of color. This can also be due to the trauma of hemorrhage caused in the down layer of your nails. This trauma is caused by dropping something on your toenails and ruining your manicure or picking it off harshly and it turns brown. The proliferation of the bacteria or fungus might have occurred in that particular area which produces the pigment of green or black but mostly it is brown.