To answer the question, while it is usual to select flashes in a color to tone with the tartan of the kilt, it can also look quite good to match the flashes to another color in the outfit, and even in some cases flashes are worn that match no color in the outfit at all!

The important thing is to consider the whole outfit. It is always safest to select a flash color that tones with the tartan, and so this is generally reccomended. Why? Well, when a salesperson is helping you select your flash color, they will know the color of your kilt, but they won't neccesarrily know what you may or may not be wearing with the kilt at any given time. Thus is is safer for them to reccomend a flash color that they know will go well with the kilt, no matter what else you are wearing with it.

Also, the manufacturers of flashes (the nicer ones, at any rate) choose their colors specifically to tone well with the color ranges of most of the major tartan mills.

However, there is absolutely nothing that says one has to wear flashes to match the tartan. As you suggest it is also quite permissible to wear flashes that match another color of the outfit. Like I said, you must consider the whole outfit and how everything goes with everything else. You may have a color in your shirt, sweater, jacket, or even bonnet that may not match your tartan colors, but tones well with it, and you may choose flashes that work well with that color.

I'll give one example. I sold a gentleman a plain charcoal grey tweed kilt once. He selected a matching charcoal grey tweed Argyle jacket and grey hose to wear with it. When he asked for advice on flash color, I asked him what shirt he was planning on wearing with the outfit. He said he had a burgundy shirt in mind that he thought would look good with the tweed color. So I sold him a set of burgundy flashes to tie in with the shirt.

Sometimes one even sees flashes worn in a color that doesn't appear anywhere else in the outfit. I'm thinking here specifically of red flashes. Often these seem to be worn universally, even though there may not be a speck of red in the rest of the outfit. You see this especially with the dark Black Watch tartan. Look at this gent's uniform:
http://home.stny.rr.com/the7thblackw...ing%20good.jpg

There is no red in his outfit at all except for the flashes, and they look quite good. They add a bit of color and brightness to the uniform.

Now, all this does not mean you should just wear whatever color flashes you want willy-nilly. But you can and shoudl consider more than just the colors of the kilt itself. Keep the whole outfit in mind and you can open up another whole range of possibilities.

Aye,
Matt