Been thinking of making sights for a muzzleloader, but this is a generic open sight question.

Most O.S.'s are iron or synthetic nowadays and typically blued, black or otherwise dark colors.

Would there be any other issue with lighter colored material being used provide they were matte or otherwise non reflective?

For instance, peep sight from matte brass or other lighter colored material.

Brass etc can be a f*cker in sunlight.

This is far and away my favorite front sight with an aperture.

I prefer a gold bead, like on some of the S&W target revolver sights.

In regards to peep sights, I don't think a lighter color helps any. Your eye centers the opening because that's where the most light is concentrated. Williams make a "Twilight" aperture with a ring of brass around it. I've got one around here somewhere, I found it too distracting.

RWE,

While I usually prefer a front sight like the one Steelhead posted when using an aperture sight, have also had good luck with traditional brass and gold beads by filing them at a 45-degree angle away from the rear sight. This leaves the flat surface slightly matte-finished, and prevents off-center highlights on the bead. This also works with white beads, but they aren't as critical about highlighting.

Have also used fiber-optic front beads of various colors with good success, but some are relatively fragile.

I will never use an aperature without the brass insert. It completely eliminates the "fuzz" factor for me, and my eye interprets that as being a much brighter sight picture overall.

I paint mine with orange nail polish, it's what I use on my nails.

I'm partial to a gold bead, or gold sourdough- as long as it's gold and not brass or base metal. (I feel that gold has a certain luster that works nicely in a wide range of light in the woods.) I routinely knock beads off of guns I acquire and replace them with gold ones- a small chunk out of the old wedding band, a little solder, and a jeweler's file and presto, a new bead that won't dull or corrode.

Another vote for Steelhead's front sight.
Mine is an AO. Best I've used in low light, and seems to tighten my groups even in direct light (focus, I believe)