Efficiently read the average color of the screen content rendered by XBMC
You should take a look at the source code of Boblight.
Extending Boblight seems like a viable alternative (if it does not support what you need already!).
If not, look at src/clients/ folder. boblight-getpixel.c (for MS Windows) and boblight-X11.c are 'pixel grabbers'; standalone programs that do exactly what you need, and then communicate the grabbed color to the boblight server.
In boblight-X11.c you have examples of using XShmGetImage or the slower XGetImage to read portions of the screen using X11/extensions/XShm.h, a portion of that code does:
[...]if(!XShmGetImage(dpy, root_win, xim, 0, 0, AllPlanes)){ sleep(1); pthread_mutex_unlock(&grabmutex); return;}XSync(dpy, True);for (x = 0; x < width; x += xadd){ for (y = 0; y < height; y += yadd) { pixel = XGetPixel(xim, x, y); RGB[0] = (pixel >> 16) & 0xff; RGB[1] = (pixel >> 8) & 0xff; RGB[2] = (pixel >> 0) & 0xff; /*add it to the RGB array*/ boblight_add_pixel (&config, RGB, x, y); }}[...]