My impression, increasingly, is that the most versatile sports jackets are in 'nature' colours like brown or variations thereof. Give you already have a blue SC, if your Harris Tweed SC is in a dark brown, I would go for a beige-and-brown Glencheck. That said, the next, for em, would be a mid-grey in either donegal or herringbone so it's a pretty close call...
I'll tell which commissions I have found less useful than I expected. Very light grey jacket. Sage-green (as opposed to a more saturated green) jacket..