Back to the topic at hand.

I personally perceive modern colored kilts as more formal than ancient or weathered kilts as the bolder colors make a nice contrast with the black jacket. I have also noticed that the heavier the material the nicer the kilt looks to others, so heavier kilts are perceived as more formal.

For a wedding you would probably want the heaviest full (8 yd) kilt that you can afford in the closest tartan to the Brides colors that you can stand after all you will have the kilt for a good long while.

Also, unless most of the bridal party is wearing highland dress, you can get away with being more formal than the occasion would normally allow (e.g. a prince charlie jacket at an afternoon wedding or a Montrose Doublet at an evening wedding). After all you are going to stand out like a peacock amongst the hens, enjoy yourself (just remember that pictures last for decades)