It's just an expanded color palette, which includes green, blue and brown tones and a full black gradient which offers far more variety than the original Build engine titles.
Duke 3D and Shadow Warrior, for example, have four hues with 32 shades, while the rest of the hues only got 16 shades. Blood, on the other hand, had one hue with 32 shades. Ion Maiden's palette is more visually adventurous even if it's less consistent than the older titles. I thought the screenshots would demonstrate the richer range while being true to the limitations expected of the classic Build era titles.
I see your point though, as it's probably a bit nerdy of me to be excited over!