Bright day, bright color. Higher background contrast. Larger patterns will produce on these days. Big trout and wild fish are photo-sensitive to sunlight. It is well documented that big browns feed nocturnally.
Dark days are great for big dark baitfish patterns.
Bright sunny day, you may want to try something with crystal flash in the pattern. Drop a hook size or so for this situation. Silver or gold tinsel bodied flies work well on sunny bright days. We can get very technical about this also. Examining the undersides of streambed rocks for nymphs, seeing what is holding in the vegetation along the banks, using a specimen net to skim the water for exacting information.