Is there a standard for marking wires negative/positive?
If one of the conductors is intentionally grounded then the National Electrical Code requires that it be colored white or gray. Refer to Article 200 for the grounded conductor and Article 690 for the PV system.

For ungrounded DC conductors supplying circuits operating at more than 60 volts the 2017 National Electrical Code requires red for positive and black for negative or other methods that are described in Article 210.5(C)(2) for branch circuit wires and 215.12(C)(2) for feeders as ThreePhaseEel points out in the comments below.

Good luck!