Along with the color of the Power switch (amber indicates some type of motherboard or component on the motherboard problem) there are diagnostic lights and beep codes.  HERE is the Dell manual and it details the diagnostic lights and beep codes.

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As you seem to have a know working power supply and the diagnostic lights are not lite, if you are comfortable around computers, you could try the following:

Remove the power cord, hold power button in for several seconds to discharge the residue power.

Remove all the PCI cards, the video card [if applicable], memory, all peripherals, disconnect the data and power cables to all the drives, check that the front panel cable is connected to motherboard, with nothing else connected to the system, reconnect the power cable and power the system on.

If the fans spin up and there is no change in the diagnostic lights, or beep sequence, then it would appear that the motherboard has died and will need to be replaced.

Bev.