That's probably because of a color shift, which you can tweak with color balance in post-production.
Some people have indicated the impression that ND filters help with saturation - I haven't found this to be the case, though the introduced vignetting will reduce exposure around the corners, which may give that impression (underexposure = deeper colors, fact of life).
Anyways, just a thought on the last frame - I personally feel that it may be better to exclude any building altogether, if your intention is to focus on the abstract pattern in the structure. The introduction of the building (Ritz Carlton) as an element merely opens the door to an impression that you have made a mistake in keeping the photograph level. When I first started photography, someone told me that if I wanted to tilt something in the photograph, it was best to make it significant, so as to avoid inviting doubt that it was a mistake. It's something that I've found very true and is probably applicable here. Cheers!