Welcome Saecoman. There's some nice tones in the Royal Stewart namely the blue and since the white and yellow are so close together you can use off white (maybe). The other option is a green, maybe the lovatt green as it's the complimentary to red. So I was thinking off-white (cream) and blue flashes. Or white with tartan flashes, but since the kilt is such a stunning and bright tartan you should ground it a bit with some darker colours; blue hose and tartan or white flashes. found a good link in your continent with these:
http://www.buzzkidderoriginals.com/hose/kilthose.htm

Let us know how you go.