if you have a 903-A, the cabinet wire colors should match what the schematic says. If you have a different 903 model, then the schematic for the 903-A may not apply and you have a lot more work verifying where wires are going.
the hopper plug would also match the schematic plug chart and the wire colors between cabinet and hopper plugs match at each pin position.
if your hopper has different color wires, it's probably the wrong hopper. Is there writing on the hopper and is the payout counter M-645-144?
if the hopper is wrong, it may still be possible to use it ... it's just work to see where each wire goes and mate it to the appropriate cabinet wire. Fortunately, bally mostly kept the same wire colors for the same functions, so most wiring is just match-the-colors. You still need to verify tho.