With that shirt, my FIRST impulse is gray silk tie (and gray flashes too).
You could also do green or red/orange (which ever is the RIGHT color), BUT these need to be fairly close in tone to the kilt (the red needs to be REALLY close, but green can be a bit more "off"). I'd also get flashes in the same color as the tie. The eye can REALLY tell differences in red and is pretty unforgiving to the tone difference, while it "gives" more on green before it "looks bad."

The significance of the tie and flash match is to give an upper and lower "tie." A black "dress" shirt and black hose will look GREAT with matching "flashes" of the same color. The color would also be good if it tied into the rest of the outfit coloring as well. The gray would tie into the sporran (at least) and red or green would tie into the kilt.

I might even try something similar myself now . . .
Thanks for the idea inspiration!!

(Now, back to sewing my 16 oz Wallace kilt.)