Back in '89 I was SLF in a Wardair A310 out of YYC on a fine day. Taxiing out I noticed frost over the wing tanks from cold soaked fuel and considered if I should make a fuss, but in relation to the total wing area and being well back from the leading edge, I decided it was acceptable.
Back at YYZ, I sent a note to ops and received a reply from the CP that the A310 certification allowed frost over the wing tank.
After the Dryden report came out, I believe Transport Canada made regs requiring an absolutely clean upper surface irrespective of manufacturer data.
If the manufacturer has done the test flights with frost over the tanks (as well as under) and has properly accounted for an engine failure, a thin frost over the tanks can be acceptable in operation.