Orange light on 745 MB is normal.  Just lets you know it has power, but isn't really a much of a diagnostic tool. As for power light:  Solid amber can mean bad MB.  Blinking amber can mean bad PSU.  Not add any confusion, but fans spinning and nothing else working can be classic of a bad PSU.  Bad PSU's can still spin the fans. Do any of the numbers on the front panel stay lit? Replace CMOS battery with new one first.  They're good around 5 years. Next if no go, try reseating RAM.  Then CPU. One can try the Dell BIST Test.  However, BIST Test won't apply if there's no green l.e.d. and test button on PSU.  Then I'd have to suggest trying a PSU tester. 
This OptiPlex 745 has been out of use for a number of years. When testing to see if it worked, it turned on immediately, fans whirring, but no display is sent to the screen. A solid orange light appeared on the power button and the motherboard. I pulled out the CMOS battery and put it back in and reset the jumper, but still nothing. The monitor and VGA cable are new. I've read posts on other machines, but none on the 745. Does this solid light indicate a dead motherboard like the rest?
Solid orange light on Optiplex 745 motherboard