I say be bold and go with yellow - they'll really help bring out the thin yellow lines in the tartan. I can't wait for the pair of yellow Lewis hose from the Scottish Tartans Museum that I ordered recently.

As for flashes, I vote you go with a colour not in your tartan for some variety. If you must, match them to your shirt/sweater.

In the end, it really depends on what you (or your significant other ) like the look of!