According to the American Rabbit Breeders Association (ARBA) and their Standard of Perfection (SOP), Holland Lops are divided into the following 8 color groups:
Agouti, Wide Band, Self, Shaded. Tan Pattern, Ticked, Pointed White, Broken

Within each group is a collection of similar genotypes (genetic code), but the phenotypes (observable features, such as color) can appear to vary widely.  In an effort to make it a bit easier to understand, know that every bunny color is either BLACK (B), BLUE (B_ dd), CHOCOLATE (bb), or LILAC based (bb dd).  Blue is the diluted form of black, and lilac is the diluted form of chocolate.  Diluted colors as well as chocolate are more difficult to achieve than black varieties since these are recessive traits (a recessive gene has to come from each parent in order to be visible, so the mom and the dad would have to give the baby a dilute "d" gene to make it "dd" and/or a chocolate "b" gene to make it "bb" and appear chocolate).  Take a look at the following table listing SOME of the colors within each of the 8 Holland Lop groups and whether they are black/blue/chocolate/lilac based.