When a customer asks to show a little Tartan with their solid colored kilt I often sew a piece of Tartan fabric in to the space between the last pleat on the right and the reverse pleat.

Now, Understand that my kilts have most common, uni-dirctional knife pleats, as found on most kilts. With the Reverse Kingussie pleats on a UK this is not possible.

I have also done the insides of box-pleats with Tartan.

Your idea is not unusual.