The only thing different in fwd and rev. is the position of the contactors. The normally closed tips may not be conducting after truck warmed up. Check for proper contactor operation. Poor spring in Fwd contactor will prevent normally closed tips from conducting.
The box color is not important. If it is a 020 guts or 350 is what counts. Early original boxes were red. If it has a small black slide switch on the mother board it is the newer 350 box and is not interchangable with the 020 box which does't have the switch.
If the rev contactor is not coming in but the fwd is. I would check the Fwd switch in the controller. It may be hanging and the black box will not let both contactors pull in.