The brown tail, and the rest in normal color, seems to be video card driver problem.
Try to switch the rendering engine. just open ogre.cfg in notepad and change the first line to:
Render System=Direct3D9 Rendering Subsystem
if doesn't work, try to update your graphics card driver.
if your problem is low FPS (frames per second), try to change the video setings at ogre.cfg
for better video performance, you can change the video mode to Full Screen, and at 640 x 480 @ 16-bit colour.
if doesn't improve much, maybe is time to change the video card or the entire computer with a better one.