Black and White books are printed on 50# white paper and uses inkjet technology.
Standard Color uses 50# white paper and uses inkjet technology.
Standard Color Heavyweight and Premium Color Heavyweight both use a thicker 70# white paper which has better page opacity.
The primary difference between Standard Color Heavyweight and Premium Color Heavyweight books is the ink and process in which the books are printed.
Standard Heavyweight Color uses inkjet technology. This option offers lower cost but the colors won't be as sharp or bright as a Premium Color book.
Premium Heavyweight Color uses a digital offset press. This option offers superior color quality but is more expensive to produce.
Here's a visual comparison of Standard Color and Premium Color: