Solid for a timeless enduring look (especially if it's someone's first or only mini) then tri as a second - or third! - for a more trendy look (colorblocking).
Personally solids for my wardrobe (but I'm a guy so more than 1 colour for a Luggage is just too fancy). The tri colours that caught my eye is the Slate Blue from Summer 2011 and the 4-Colour from Winter 2011. I think I'm more into bi-material leathers for the Luggage styles especially in Python...