There are a lot of nymph patterns out there, and you can divide them into two categories: those with flash, and those without it. Now, what do I mean by flash? Some patterns have flashy material tied in, and others have beadheads that shine and flash in a similar way. To fish, that flash can resemble a number of things. In some cases, the flash mimics the air bubbles that help hatching insects ascend to the surface, and can be a trigger that draws strikes in strikes. It could also pique a fish's interest, or look like the shimmering side of a fleeing baitfish. Other times though, that flash may signal that the fly is not a natural, and will give a fish cold feet.
When you’re stocking your fly box, it can be easy to overlook the subtlety of nymphs with flash and those without, but it is of critical importance. Make sure you have a healthy selection of more natural, subdued patterns, along with those that have a bit more flash. There's nothing worse than realizing that the conditions call for a certain type of pattern, only to pop open your fly box and see nothing of the sort. There are days when the fish are honed in on the bright, shiny patterns, but other times they won’t go anywhere near them. Make sure you're prepared for either contingency so you don't end up kicking yourself out on the water.
A couple weeks back, I was fishing a small, clear stream on a particularly bright, bluebird day. Oh, and it was high-noon. Not my best, I know.
Not only did I have a hard time approaching fish without spooking them, but I also made an error that was far more controllable - I got out on the water and the vast majority of the nymphs I brought with me were tied on beadheads. Those that weren’t, conveniently enough, were flashback patterns. A crucial oversight on my part, the bright sun and clear water led me to watching several trout aggressively approach my fly, only to turn away at the last moment. If you’re lucky enough to watch your refusals (as painful as it may be), this is usually a good indication that it’s your pattern, not the fish, that is the problem. After going through numerous fly changes (and moving down a tippet size), it occurred to me that it must have been the flash that was doing me in. The fish were interested when they saw the fly drifting towards them, but as soon as they got close enough to examine it, the sun would reflect off the beads just so, sending them away. While I can’t say that bright sun will always reduce the effectiveness of flashy patterns, the combination of clear water, sun, and slow current speeds certainly gave fish enough time to get a good look at my fly, and ultimately reject it.
The moral of the story is that just because you've had success with flashy patterns on certain days, does not mean that you should cram as many flashy beadheads as you can into your fly box. Sure, you'd be ill-advised to have too little (more often than not, I am, indeed, fishing beadhead patterns), but make sure you have your share of natural patterns too. Generally, you don't realize you need them until it's too late, and at that point, it's just that... too late.