I think there is some confusion in terminology here.

Garters are the things that hold the socks up. They can either be the traditional woven or knit bands which are wrapped around and tied, or they can be modern day elastic garters; either serves the same purpose.

Flashes are the colored ribbons that are attached to elastic garters. These are meant to emulate the ends of the tied knot of the traditional garters. So it is not correct to refer to traditional self-tie garters as "flashes." They neither have nor need flashes.

HRH the Duke of Rothesay does not wear fancy garters over top of his hose. I always see him with very traditional plain red garter flashes. I think what you are referring to are the check patterned top hose he is often pictured wearing. This is not a separate item worn over his hose -- rather the pattern is knit into the hose themselves.

As to what to do with too-long hose, this depends a great deal on the hose in question, and just how long they are. There are many different styles of kilt hose. Some are designed to be worn with the tops turned over multiple times (such as the pipers hose already mentioned). This is intentional, to give a top with 3 or 4 layers of thickness. Other hose have a pattern to the cuff that would not show properly if it were folded down multiple times. This does not necessarily have to be an embroidered motif or a knit color pattern; it could be a cabling pattern or some other pattern in the knit. In this case it is best to put a little accordion fold under the cuff -- folding the hose down, up, then down again so that the proper pattern of the cuff shows.

Some kilt hose are designed with a longer or shorter cuff (turn down) to them, as well, which means that more or less of your garter or flash may be showing. You can also adjust your garter if needed.

Really none of this is rocket science. Just fiddling with it for a few moments to get it looking the way you want it to is generally enough. But there will not be one solution that works right in all instances, due to the different make of hose and garters out there.

Good luck!