It's safe to say that a fly angler could spend a lifetime trying to figure out everything there is to know about the midge and it would be the kind of thing that would lead some of us to drinking... more.
After all, there are several thousand species in the midge world. Then there are colors to consider, along with size, and different life stages of this insect.
In short, a midge is a two wing fly, belonging to the classification of Diptera. They are a quite small fly, but often thick in numbers. Chironomids are most likely the predominate presence of midges and they thrive in cold water environments where trout also thrive.
As far as colors to use, my experience, although limited, has been that black, olive, creme, and cinnamon will work at times on Blue River. But, the guessing game is which color to choose. Black is a difficult color to beat at most times I would suggest. I've only seen a trout's stomach pumped one time in my fly fishing life. As the gentleman who did the pumping emptied the deposits in the palm of his hand there was nothing but minute black midges. After seeing these creatures it was easy for me to go to the fly box and match exactly what the fish were keying on - a size 22 - 26 black thread midge.
I think that when we are tying our midge patterns we quite often tie them thicker than they should be. Most things in nature, as far as trout food, are usually smaller than we envision.
Size - oh how important is size? Extremely important, but again, it is usually a mystery to us while we are standing in the river. My approach these days, when strictly fishing midges, is to give the trout choices. Choices by way of showing the pretty fish three different sizes and stages of the midge. But again, the color is a guessing game.
And, then there are the stages that range from larva to pupa to emerging (or what I call shucking) stage, to the adult. Of these stages, I'm convinced that the shucking stage is the most important. This is where the midge is desperately trying to escape the shuck, to fly away, and is the most attractive and opportune offering for the trout.
Usually on Blue, when I get to the river early it's all streamers. Come mid-morning if the streamers are faltering, a switch to nymphs will be made. And in the afternoon, it's time to go to midges. However, if at anytime, there is evidence the trout are keying on midges, it's time to rig up a three midge offering.