When you only include images (no hot images or disabled images), disabled toolbuttons show the image still colored, but blurry and the caption is gray which is right.
If you include disabled images, when the toolbuttons are disabled, the disabled image appears; however, the caption is still black or normal. This can lead to confusion on the user's part.
I have attached your MDIdemo with 2 image lists and a toolbar with toolbuttons added.