No, greyscaling the image is too much in the way of postwork.  If we allowed that, we'd have to allow other hue shifts, since it's running in the same vein. 
If you're looking for a black and white image, it is possible, with material settings, to make your figure grey, it just takes some material room fiddling ;)

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Knowledge is knowing a tomato is a fruit. Wisdom is not putting it into a fruit salad.