Generally speaking in highland wear the red is for formal dress and the green or hunting is for daily wear. Or in some cases the dress tartan is for formal wear.

The idea is that in daily wear you are wearing the color that blends into the bushes etc. and also that doesn't show dirt as readily, but for formal occasions you can dress more brightly.

However, in non-kilt wear darker colors are more appropriate for formal wear, so it may be appropriate in some cases to wear the darker color tartan for formal wear. Especially if you want to minimize how much you stand out from the pants wearing crowd.