Red will be the most discreet, and will help maintain your natural night vision best. But this also means they will be the dullest.

The white are good for true color ID of objects and environments. They are the second brightest of all your choices.

The blue are in fact the brightest of all the three in my experience. It also throws light the furthest of all three. If this sort of thing interests you, blue also makes blood stains stand out.

I went with the white myself. I liked that it gave true color to the environment I'd be moving in, and that they were brighter than the red, but duller than the blue.

There is no wrong choice really. There may, however, be a choice that is best for you based on the above.

And remember, you can also get a tip off color filter for the main light. They too come in red, blue and infrared.  For instance, I have the white LED's but have a blue filter for the main light. Just throws a little more variation into your system and its capabilities.

I knew that the M900 series had the nav LED's, and the Surefire page says "some" of the fore end lights have them, but are you sure that the M500 is one of them? I haven't seen one with them and the web page is not very specific.

New arguy summed it up quite nicely.  I prefer white over blue as it provides correct color identification.  If you want the best of both worlds, you can custom order one with separate led color - e.g. white on one side, blue, red, or IR on the opposite side.  Which ever Led you do not wish to use, simply cover it up with the supplied rubber boot.  

If you get the waterproof version it only comes in white and this is what the Navy is ordering for...well Navy guys that use M900s on M4s  

If its good enough for them its good enough for me.  Also they have little nipples to cover one of your LEDs if the total light is too much.

Dont get red LEDs if you have a CompM, Eotech, or ACOG with red retical.

Dont want your LED "running lights" washing out your primary optics reticle.

Wow, great thinking, I would not have thought about that! Did you learn this lesson from experience?

New-arguy,

AR15FAN is right , the white or blue leds are the best , even if I find two of them are too bright , I use to cover almost one with OD tape.

I read the Surefire produce a M900A model with IR leds too,they'd be nice to light up dark rooms without the big power of the M900A / #FM13 main light.

PP out

Some corrections:

1.  Red LED's will not wash-out dot on M-68 (Aimpoint)...give it a try.

2.  LED's (navigational lights) are not designed to illuminate your target.  Thats why Surefire put that big main light on the end.  They were designed to give you enough illumination to maneuver during limited visibility without NVD's.  If you need more light to maneuver...go with white or blue.  One Red LED puts out plenty of light to maneuver.  Even that is very easy to spot at a distance.

RLTW

Just curious, does anyone have or seen the waterproof version of the M-900?

No but I am getting it for Christmas.  Surefire told me I might have to wait for up to 6 weeks when I order it because the do the "proofing" in the tanks in groups and they need to have several to go at once because of the cost.  They are supposed to be the same except for double O-rings, Dielectric grease on all fittings, warrantied against damage from immersion in salt water.  The surefire technical assistance guy also told me they hand pick parts that are tightest in tolerance so it will be sure to pass the immersion test or they have to make a new one and try again.