Excellent series, one of my absolute favourites and perhaps the show to sell anime to me.
Holos hair is one colour.
I must also note the sixth novel has just been released in English and is the first to cover events after S2 of the anime excluding one novel whom was excluded from animation.
I personally prefer Ami Koshimizu's Holo as I believe she shows a greater complexity/range of emotion in her voice compared to Brina Palencia's take but I have to admit the English Lawrence is excellent from what I've listened to, having owned the US releases for quite some time.
S2 is also excellent but focuses more closely on a few events as opposed to S1s slightly wider travels, giving it a different feel albeit still excellent.
NOTE TO REVIEWER:
YOUR LAST PICTURE ABOVE IS SEASON 2 SPOILER
Typing from my phone so apologies about a slightly fragmented post.