So I created a test to characterize the non-linearity of colours with this screen (the test is not released here), and then I made a daemon that corrects that non-linearity kind of in the same way as Photoshop's Curves thing. What I'm releasing here is an early version of the daemon alone, with the correction values that I found with my test hard coded in it (should be correct for all MKII's I suppose).
So here's how it works, download the file attached to this post [attachmentid=329]. Copy "colourd" and "colourd.gpe" in the same folder anywhere you want in your GP2X. Launch colourd.gpe. Now do stuff you usally do, play a game, watch a video, etc.. The difference is most obvious on dark stuff, like Quake or Duke Nukem 3D, or also on bright stuff that's usually burnt (looks all white as it shouldn't). Notice that the colour correction lasts until the GP2X is restarted. If you want it again once you've restarted, just launch colourd.gpe again.
For those who would wonder, this thing has absolutely no speed impact since it's entirely based on setting a few hardware registers once. Not a single hundredth of a FPS will be lost in your favourite game because of that.
Now what I'd like you to do is for you to try to see the difference between before and after, so that you can say whether you like it or not, which will decide whether or not an improved version of this will be implemented in Open2x.