Is it possible to draw an image and then turn the image to greyscale? I don’t think it is possible in my current version .34, but maybe this option exists in newer versions?
You can draw a new (greyscaled) image on top of the colored one. This would require a second image file of course.
What I do to make an image look different is to draw a semi-transparent rect (e.g. using NuiDrawListPolyline and color NuiColor(0, 0, 0, 96)) on top of the image. That gives it a darkened or disabled look. The new dev patch also has NuiDrawListRect() which makes it even easier.