HBP is full of color. Reds, golds, blue, green, greys, sepia, browns - it's all there. But they aren't bright and the contrast isn't popping. Like Tim said it's a soft image with bloom and it's meant to have a painting-like quality to it. I can see why it isn't everyone's cup of tea, but it isn't bad cinematography. It takes a skilled cinematographer to pull that look off.
If I had to pick my least favorite cinematography in the series it's easily GOF. After Michael Seresin's brilliant work on POA, it was a bit too bland and dreary, and is the only cinematography in the series that I really do not like at all.
Sławomir Idziak brought some life back to OOTP but I'm not overly fond of the abundance of pinks and blues in the grading but the pink makes sense. Umbridge's favorite color is pink and she represents the Ministry and it's slowly permeating everything at Hogwarts so it makes sense that varying degrees of pink be in the image.
If I had to rank the cinematography of them all for me it'd be:
POA
HBP
PS
COS
DH2
DH1
GOF
Hang on, so he's directing both this and Tarzan for 2016? There's no way he'll have time for both, but the latter has had a release det and cast for months, so has he dropped that for this?
He is currently filming Tarzan and will start filming Fantastic Beasts in 2015. He will have the same amount of time do Fantastic Beasts as he had to make OOTP, HBP, and DH 1&2. And because both projects are at WB he can work on Tarzan post-production while prepping Fantastic Beasts. It's entirely doable and something he is used to at this point.