The whole point of a solder mask is that it's typically applied using silk screen techniques and it covers the entire back of the board except for the pads where there are to be soldering operations. The mask keeps stray solder from bridging across close traces. So spraying it on after assembly wouldn't make a lot of sense.
Sounds more like you're wanting a conformal coating to protect the finished board from corrosion. You can get that using an acrylic spray (e.g., Krylon Clear Spray here in the U.S.). If you insist on green (I've also seen red and blue solder mask), just find a can of green spray paint and use it. I would test it out first to check on conductivity when it's dry. Also, any coating can shift stray capacitances very slightly so that you wouldn't want to align a radio circuit until after the coating is dry, especially if it operated in the VHF or UHF range.