Using wool is the closest to 'matching the hatch' in gear fishing for salmon in rivers. We must remember that salmon don't actively feed in fresh water, so their bites are essentially out of aggression, trying to destroy competitors' eggs. You figure what species of salmon/trout is in the system, what size their eggs are, how long have those eggs been in the water, would the eggs still be translucent or milky...and based on that you pick your wool colour and size of the tuft. If your tuft is bigger than your pinky finger nail, you are not imitating a trout/salmon egg.
Interesting to note how the different species can be triggered by the different colours. Springs and chums can be had on all kinds of oddball wool colours, including green, blue, chartreuse, purple etc. Coho on the other hand, dont seem to respond consistently outside of the 'natural' shades. In context to what Milo wrote, if cohos are reacting in an evolutionary tactic to eliminate potential competitors, then presumably the springs and chum are reacting in a more territorial strategy where their aggression is triggered by something they don't like invading their personal space.
Though cohos can be taken by spinners, jigs, etc. where the blue/green colours can be very effective, in which case I guess they are hitting because of their baitfish prey/predator instincts even though they are not actively feeding in freshwater. If you are are going to go after coho with the egg-mimicry route, then I'd definitely agree to stick with the natural shades.