Keeping to the colors of the tartan, with the darker hose, the yellow is the only color option that will really contrast. You could certainly use the other colors, but they will tend to blend with the color of the hose.

Lighter colored hose would certainly open up the options quite a bit. Another option is the aforementioned tartan flashes (only in the same tartan as the kilt please).

I have seen several pictures where some have worn flashes of a completely different color, often red. While I didn't think the combination looked awful, I much prefer to keep the flash color coordinated with the kilt.