Developing a Mac Utility to Alter Screen Display/Output

I want to build a Mac OSX utility that alters the appearance of the screen on a level similar to OSX's native "White on Black", "Grayscale" and other contrast settings located in the Unversal Access area of the System Preferences. To be specific, I want to adjust the display to give off a darker appearance of the screen, so that it's not so bright at night because even the lowest brightness of the MacBook is still way to bright when being used at night. I haven't seen any Mac utilities offering such a capability so I'd like to create my own if possible (unless by chance you have come across something similar). I have experience creating Mac apps, but nothing of this caliber.

So I'm wondering, how would I go about creating such a utility? I know there are apps that require "Enable access for assistive devices" to be checked in the Universal Access preferences so they can alter windows of apps and so forth. I assume such a utility would also require this setting to be enabled in order to access some kind of low-level开发者_如何学JAVA accessibility API, but I can't find any relevant information on the topic. I tried to hunt around on Apple's OSX developer documentation to see what could be done with such settings, but I didn't find anything useful. Is such a feat even possible to pull off? If so, how? If not, why? The solution would only need to support Snow Leopard, if that helps. :)

Shady by Matt Gemmell does exactly what you describe (open-source):
http://mattgemmell.com/2009/11/02/shady-for-tired-eyes
It's basically a transparent window on top of everything.

You can also darken the screen by altering the color profile.