I'll toss in my two pennies.

Since it sounds like the only level of dress you'll not have there is formal. I personally would go with your Docs, blue flashes to match the blue in the tartan. (I confess I had to look it up.) With solid black shirt, like a long sleeved T shirt. If it's cooler at the time of the gathering, I'd go with a long sleeved solid black without a collar. If it's warmer, loose the long sleeves.

You'll be a little of both. If you wear the flashes, it'll be a little bit of color and more formal, but the solid black is more casual, but will still be more of a relaxed look. You're already going to get more attention then if you wore pants, so subtle will look good IMO. Sort of like the jeans, solid color T shirt and brown corduroy jacket combo I wear when I'm going someplace and want to look somewhat dressed.