The 5-pin (2x3) header (2 mm standard) on this motherboard uses the Dell P/N: 030WGC cable for Power Switch, Power LED, and Diagnostics.  This cable uses a momentary switch and a multi-colored (white, orange) LED.
Power Switch (momentary) function: Black and Yellow wires.

Power bi-polar LED (White / Orange): Red and Green Wires

This Dell Optiplex 0x20 MiniTower series uses White - Normal and Orange - Sleep or Diagnostic (blinking).

I believe that the red and green wires are actually different voltages for each color, white being 5v and orange being 3.3v.

A different theory follows a more than common pinout for standard RGB LED strips, where each color is a ground and a stead 5v is applied to the strip rather than to each LED.

I will be testing this later tonight as I am putting an O390 motherboard in a standard atx case.