There’s a good book called Feeding Time. It conveys what causes fish to strike. In particular, the book documents how certain traits have to be in a fly for a trout to commit to taking it.
In my opinion, there is a hierarchy of strike triggers: size, then silhouette, and then, color. Size is pretty obvious: you want to try to match the size of the insects floating in the water. Silhouette is all about the profile of the fly. Most people tie flies that are way too thick, and I inadvertently do that, too.
And, then comes color. I think matching the precise color of a natural bug is really hard to do. There are multiple shades of yellow for a sulphur, for example. So, I do my best with what I can get. I really don’t want to buy too many bags of dubbing.
One thing I do is to go for really wacky colors. For example, I have some nymphs that are tied with blue thread, based on a blog post I read (photo up top).
On some days, the trout want a very boring fly. Other days, they want something very bright. So, I normally tie normal flies, but always have a few available that are just unusually colored. I think at this point in the season, many trout have seen all sorts of flies. So, it pays to throw a curve ball.
So, test your imagination and pick some wild colors and have a few of those flies in your varsity fly box. I think you’ll be surprised, pleasantly, with the results.