I would suggest you take a look at " Competitive Shooting" by A. A. Yur' Yev. This is the Russian Olympic shooting book. They tried many different colors and shapes of apertures. They concluded that no filter was best. I have use Gehmann irises and now I favor a Merit disc. It generally offers a smaller aperture which works best for my old eyes. I have used a green filter on bright days and a yellow filter on over cast days. I shoot HP and smallbore.  Good shooting!
Kurt

Depending on your ability to hold a post can be handy in offhand. If you are fighting a wobble area, a post 
offers a little more control as you drift to center. Most of the time, I also like an aperture. As Schuetzendave says your front aperture focus should be sharp and accept the less sharp target. When you are young your visual acuity, permits your focus to move rapidly from front sight to target. You loose that ability as you age.
  Kurt

Plastic front aperture. 

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Clear.  Best I ever used.  A very calming front sight picture.  The beveled hole and the non descript color worked well for me in all light, but try them all, at different times, to find what works best for YOU.
Cary

I use a tube sight with an clear adjustable Anschutz front and an Anschutz rear with colored filters.

The filters are rarely used but the light grey and the light green can be helpful on occasion. Green is the color complement of red and will substantially darken the red bull.  This helps me when mirage is up.

Light grey helps on really bright days.

The only way to determine if a given filter is helpful or not is to try them under varying conditions of light and mirage.  Not everyone "sees" the same and what works for one shooter on a given day may be the kiss of death for another.

JackHughs

Filters are a very individual choice. When I shot a lot of small-bore prone in Australia my club owned Anschutz 54 Super Match had all sorts of filters available colors polarized etc. Club shot under several light conditions, Match every Wed at 7 pm, daylight time twilight, off daylight under artificial lights.  Sat Morning match with bright sun coming from 9 o/c at times other times deep cloud shade.  Bulls eye target using good aperture sights I gave up on all the filters but did vary the aperture size front and rear often. Match time set the front according to expected ambient condition bright or dark,  every shot checked sharp front ring focus if not sharp spun the rear dial fine tuning

Post vs. Aperture is another thing. For BPCS or Lever Silhouette post is much better for placing your shots. Lever it’s required, BPCS you can use anything you want.  Indistinct aiming point picking the right spot on the critter is essential. Down side is small pin head post is very hard for older eyes to focus sharp, and get good focus on the animal same time. For a number of years I shot our club offhand iron sight matches as good as anybody using a very small pin head front post.  However last few years my eyes won’t hold good focus on the sight for more than a few seconds. Have to hold the gun eyes closed open and break as soon as I get a good sight picture. I can show you a log with every single shot missed over 5 years and probably 90 % of misses were due to fuzzy front post and bead.

End of last season loosing the 200 shot CF aggregate club championship by one target figured I needed something new.  No sense in letting eyesight dictate the score.  I might add fellow that beat me is older than I am and has the same focus issues.  He uses post and bead front too.  Backed off on our matches and in practice am trying to learn to shoot silhouettes with aperture sights.   

Finish in a 2nd post

Boats

What is an adjustable aperture front sight?  I use the clear plastic inserts in a Lyman front sight but that isn't "adjustable", it's replaceable.  Is this some new toy I need to buy

Dave

One on my prone rifle was a Anschutz.  Result is no different than changing the front Arpeture. It's just faster and can be done from position.

Arpeture I am using now for silhouette is custom made by Distant Thunder.  A bit too big am going to get him to make one a bit smaller. 

He will make anything, friend of mine has a rectangle front.  Idea is it frames Rams.   Not a success far as I can see.

Kurtz "Russian Book" advice is good. None of this is new, tested to highest levels by the Russians.

Boats