With the, unfortunately, rather loose interpretations of "formal" these days, you were told that suits would be seen. The kilt, however, does provide some interesting wiggle room. If your Argyle jacket is a dark charcoal and not too tweedy, wear that with a black bow tie and formal shirt. Since my family tartan has a grey background, I wear a dark grey PC. But I would not hesitate to wear my black Argyle to an opera ball if I wanted a different look. The chances are I will be only one there in a smart looking kilt outfit and I certainly won't be criticized for being underdressed.