If your bright screen is still rather dim, you have an AGS-001, or an original GBA SP model. If its brightness is on-par with that of a Nintendo DS screen, you have the AGS-101, or the newer of the models. By the looks of the color on the screen, you have the AGS-001, however, I can't get an exact color, because all your buttons are different colors, signifying that at one time, the case or buttons were modified. The GBA SP's buttons are all the same color on the system, although there are dark and light grey buttons depending on the system's color. Yours has a mix of dark and light grey, so the color of the case may have been from either the 001 or 101.
You can, however, cross-compare the colors in
this list with yours, and see if you can find a match. If it doesn't match those colors, there is the small possibility that it could have been a custom case as well. I'm leaning towards it being the Marble Blue color, but I'd need a better photo to confirm that.