|The orange light on the monitor is just an indication that it is not receiving anything from the graphics card in the computer.|
The fact that the lights come on and the fan spins does not necessarily mean the the power supply is working properly. The fans spinning is a good clue that the power supply may not be supplying enough power to get everything working. I would first try with a know good power supply.
But first, take out the ram and give the contacts a rub with a pencil eraser. Deposits can sometimes build up on the contacts. Reinsert the RAM making sure it is seated correctly and try again again.
You can leave any hard disk, DVD/CD drives disconnected as you should still complete a POST without them connected.
Once you have eliminated the power supply and the memory then it is probably the motherboard but test with a known good power supply before you condemn the motherboard.