Camera appearance black & white

OK know my question is how to make an effect of having not a scene because I actually want to have the scene black(dark) but I want the camera that see all in black and white. If some one don't understand me check Splinter Cell Conviction game.

For this I got an idea but I need a scripting for this. I am wandering if is there a way to do something like if the color of something lets say sky-blue is going to transform into light gray and have something like this

* white - 80-100%
* light gray - 60-80%
* gray - 40-60%
* dark gray - 20-40%
* dark - 0-20%

And the script something like this:

if(lights.inLight == false){ //inLight is a variable that is on the lights script and is static
/*the objectcolor is 70%*/ 
if(/*main color of the object is 60-80%*/){
/*color = lightgray*/ 

This idea is not to good because you will need to add this script to every object.

And another idea I have is make the camera to do an appearance of black and white but first I don't know if is possible and second if is possible I don't know how to do it.

I am not using Unity Pro and I don't want shaders for doing this.

Please answer. I will really appreciate it.

You say "I am not using Unity Pro and I don't want shaders for doing this." However, those are the only two approaches that will work. You either need a full-screen render-to-texture effect, which requires Unity Pro, or you need to change all of your shaders to incorporate the effect. If you're willing to take on the latter, this might be a good place to start.