I agree with both of you gents. Don't get me wrong I don't NOT match hatches. Especially with my olives I have cripples, emergers, comparaduns from 14-28 missing a size or two. And I have general color flies that I match to the expected hatch. For instance if Green drakes are hatching I'm no dummy, I carry a #10 2xl Green drake no sense making a general imitation for an insect that has no twin. Just like the brown drake, I guess you could use it for a big Iso but there really no other insect similar to it so there's no sense in generally imitating that.
I guess what I'm saying is I'll generally match a color and I believe you can match almost every mayfly with cream grey and mahogany...and then my olive selection.
Now caddisflies are another thing...