Artists couldn't make the gray hair look good apart from Pat Broderick. Every time Hal showed up it was like how grey his hair was changed from panel to panel. It looked terrible and that was probably why they got rid of it. That being said saying him getting older was due to demonic possession was stupid cop-out storytelling and just came off kinda pathetic. Keeping characters static like that was why I preferred to read manga when I was younger where they weren't afraid to have characters grow and change and learn over time.
So as far as the grey temples I don't care either way but since they were rarely made to look right it's better that they are no longer around. 'Course if Emerald Twilight gets a movie which may happen that might change.