If you can manage to come up with some original, meaningful, conceptual justification for turning Marilyn black and white then, yes, it becomes an original piece of art. And I can definitely come up with reasons as to why turning Andy Warhol's coulourful portraits into monochromatic images should be considered art.

The difference between Jackson Pollock and a four year old splashing paint on a piece of paper is the meaning and intent behind their creations. It's the "why" that's important. 

From what I've seen over the years art is little more than the ability to justify your bullshit, [sometimes even literally](https://www.irishtimes.com/culture/a-tale-of-two-indias-told-in-barrels-and-pipes-cow-dung-and-hair-1.1756123), haha.

Oh, and [a signature and a title](https://en.wikipedia.org/wiki/Fountain_(Duchamp)) can make all the difference too, haha.