Yea, the orange wire is just used instead of the typical yellow (12v). I have a PSU that has orange 12v lines as well.
When I use a ATX PSU to do bench testing I simply jumper the green wire to a black wire with a paperclip.. I then take my two power wires for my M2-ATX and connect them each to a 12volt (yellow / orange) wire. Then connect the black of the M2 to a ground.