Sunglasses and lens options - I actually know something about this.

I love cycling, bicycles, racing, and all that related stuff.  I have worked in a bike shop selling and counselling customers on bike choices.  I have gotten my hands dirty fixing my bikes and my friend's bikes.  However, there are lots of people out there who know a lot more about cycling than I do.  My background in eye care does give me a bit more information than some when it comes to (there are still others out there who know more than me ...).

I thought I might pass on my experience and knowledge in case anyone wants to hear my opinions on this topic.

First off, lens choice is very subjective thing.  What one person prefers as their lens of choice is not what another person would choose.  I have met people who wear one lens color for everything, and some who choose a different lens for every light condition and every activity.

In general, the most important advice is to protect your eyes from UV light.  UV is what ends up causing cataracts for most of us at some point in our lives.  As you're sitting out there on the bike for hours on a sunny (or cloudy) day the UV light is getting absorbed by your lenses (inside your eye) if your sunglasses aren't blocking it from getting there.  Luckily, most cyclists wear sun protection already.  Make sure they block 100% UV.  

Lens options come in all sorts of varieties.  This can include the lens color, lens coatings, polarization, and photochromics.  

Lens color facts / opinions:

• Grey preserves the most color perception and blocks out the most light.  However, it is a neutral filter which means that it doesn't allow the most contrast.  It is best suited for bright days that just need the brightness turned down.  
• Brown tints can block out a fair amount of brightness, but they are great at retaining contrast in cloudy conditions (provided that the overall light level isn't too low).  Green lenses tend to be similar to brown lenses in performance.
• Orange lenses are great for ski conditions - generally cloudy with a high need for contrast awareness on a white/grey background, not the best for cycling.
• Yellow lenses are the best for low light conditions like dawn or dusk.  They can even help at night time for some people because they allow light in a wavelength that approaches our peak sensitivity of 555 nm (think yellow-green, Cannondale-ish).  This means that nearly all of the light passing through the lens is at our peak sensitivity; hence, we see better.

• Clear lenses are good for night riding and gloomy or rainy days if you don't like tints.  They protect your eyes from UV and bugs, but don't affect your perception too much.

By lens coatings, I am referring to mirror and other coatings on the lenses.  This would include coatings like Iridium from Oakley.  These are filters / mirrors that reflect certain wavelengths of light to help the eyes get "better" light (and also produce a certain look to the product).  A common mirror appears blue because it is blocking the blue wavelength.  Does it make a huge difference?  Not really sure, but they do look cool.

Polarization is important to the cyclist because it is great for blocking reflected light off of the wet roads.  It also blocks reflected light off of cars windows, snow, lakes / rivers, etc.  It is usually more comfortable to wear, but it does take some getting used to because it also makes certain displays and windows (with their coatings) look odd. 

Photochromics are lenses that get darker in brighter lights (and in colder conditions).  These are nice if you want one lens that changes for your conditions without you having to actually change the lens itself.  I think they are great for cycling sunglasses, but they do not make a huge difference to actual lens shade (ie, pink lens to dark pink lens - as witnessed in my Rudy Project "racing red" pair).

I am a fan of companies that make good frames and have great lenses; especially with multiple lenses that can be easily switched out for based on the conditions.  Some brands / models come with several lenses. Rudy Project is great for this (I am surprised they aren't more popular). The Rudy fit is good, the nose pads are adjustable in most cases, and the temples are adjustable.

Rudy Project Noyz - optional lenses available

Smith Optics has a few pairs that also come with the interchangeable lenses - the Pivlok V2 is a great system, I wish they included a yellow or clear lens with each pair.

Smith Pivlock Overdrive - green mirrored lens, grey and brown lens included.

I have no first hand knowledge with many of the other brands, but I have worn Oakley a fair amount - not my favorite anymore, but good.  Maui Jim makes awesome lenses, but they lack the sport models that most cyclists would desire. 

The bottom line: get something that fits well, blocks out UV, and has multiple lenses that change out for the right light conditions, or buy a bunch of pairs with different lenses and keep the local eye care businesses afloat.