Thanks @AndyW ! True! There is no need to add the ground wire for my setup with the host board. However, if I needed to produce the mini breakout adding the ground in that instance could allow me (or others) to use the breakout independently of a host board, but still be able to connect larger/external switches and LED in an enclosure solution.
(For the yellow wire I connected to the GND pin was so I could keep the Photon + brought out LED as one package that I could easily swap on a host board.)
That's probably due to the fact that the on board RGB is a common anode and yours might be common cathode (or vice versa - not sure
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@ScruffR Mmm... OK, you are right the surface mount LED is a common anode. So does that mean my external LED's common pin should not in fact be connected to the GND pin but +3V3 on the Photon (plus obviously I should be using a common anode RGB - if I'm not. I just grabbed one with no specs.)? And if so...then does that have a repercussion on a mini breakout board option since the USB+ connection will be 5V when connected?