I believe that color matching is important in most situations dry fly situations; however, it is not a deal breaker. I would try matching the color closely if you are trying to match a hatch. Although, color matching is a huge deal if you are tying nymphs/midges. You need to get those very close.
The most important thing is getting the correct shape and SIZE. Size is probably the biggest area that most tyers do not focus on. Well, this is an area that I screw up on most of the time. I have shaking hands; so, I usually use size 20 hooks or smaller. This slight variation can reduce my strikes by 50% in not using a size 22-24 hook/fly in certain situations. Mostly; this comes in to play in areas that the majority of the food source is extremely small. Also, you need to make sure the fly's shape will give it the correct presentation that you are intending to mimic.
Don't be afraid to experiment. Some fish are keen to stimulating patterns and colors when feeding is selective and fishing is slow.
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