First, welcome to the forum.

Second, that's a handsome semi-dress (I think) sporran, and being semi-dress, you can wear it dressed up or dressed down, I'd say. There is a line of thinking here that goes for formal events (depending on the level of formality) wear what you have in the best possible way. So, if all your other kit is up to snuff on the formality scale, your shoes are polished and your hair is combed neatly, I think that reddish sporran would look smashing against the tartan you've chosen.

Regards,
Rex.