Its so easy to blame the solution however most issues of solution turning green turn out to be a storage issue.
If the lid is not sealed tight and the room warm and humid the moisture in the air will get into the solution and it will turn green.
Likewise if after a spray tan you leave the solution in the spray gun cup maybe because you have another customer that you are giving a different treatment to and with good intentions forget to pour the solution back into the bottle and seal it.
Whilst sitting in the gun cup this is easy for airborne damp air to get in and contaminate the solution in the cup which you then pour back into the bottle and seal it....making it ideal conditions for the contamination to spread to the rest of the solution.
Throw it away or use it on friends as the green tint generally is simply the initial colour guide and the tan will turn out fine....buy a new fresh bottle and keep the above points in mind and therefore avoid any complications with tan outcomes.