This variation of Rowan, which is also a color name rhyming with Joan, was given to only six baby girls last year vs. 87 boys. The Rowen version, on the other hand, was given to over 700 girls....and more than 1100 boys. And then there's Rowen, Rohan, Rohen, and so on.
While we like the sleekness of the Roan version, we'd avoid it because of pronunciation confusion. Best in this case to stick with the most widely used version of the name, which is Rowan....unless you really do mean the color, used to describe the coats of horses and other animals that are white mixed with another tone such as red or chestnut.