Smallmouth are aggressive, given the right temps. They will give you a wide margin in fly selection. I fish a handful of flies because I know they work. I think the main things to consider are:
1. area of the water column (bottom, mid, top)
2. match the local bait (size, shape, color)
3. retrieve, depending on conditions.
You can shuffle the order of importance.
Color: How often have you heard "bass like white" or "It ain't no use if it ain't chartreuse", etc. While I like all white or a combo of white and chartreuse, I'm not sure it's very important. I'll bet an all purple marabou streamer that's 2-3" long with palmered marabou or schlappen would be great for smallies because it has the right size, profile, and movement (esp. when paused).