It might make some sense to have just a grayscale eInk device, but assuming that you are going to buy this eInk device, and it would last 5+ years, then grayscale might only make sense if the particular ebooks you are looking for will just have grayscale. As the years will go on, color eInk will start to make more sense. You wouldn’t want to buy the “cheaper” grayscale eInk reader, and regret your decision 3 years down the road.
If you are reading graphic novels, then color just makes sense. The only disadvantages are price, and the cost per ebook. With the price being higher than grayscale, this will turn some people off if they are just looking in price. Also, I would imagine that color ebooks will take up more storage on the microSD card, but considering your device will come with a 64GB? card, and you can go up to 2TB?, there shouldn’t be a worry of such.