User blog comment:B214/High School DxD Hero Discussion/@comment-26468670-20171025044528/@comment-29963364-20171031023932

They first three seasons also changed a few character disigns:


 * Tsubaki Shinra's heterochromia (they made both her eyes purple/violet)

I see the new art-style to be the anime's chance to, not only right the wrongs of the 3rd season, but also stay true to what we see/read in the original material.
 * Momo Hanakai's hair color (changed it from black to white)
 * Zephyrdor Glasya-Labolas' hair color (changed from green to blue)
 * Diodora Astaroth's hair color (changed from blonde to greenish-black)
 * Millicas Gremory's eye color (changed from red to blue)
 * Issei Hyoudou's physique (it's outright stated that Issei has buffed-up from his training, but the anime just kept him "thin as a twig." I'm not saying they should make him look like a body-builder, just be consistant with facts established in the LN)