Here 8 means there is 8 combinations, but it does not means there are 8 nonequivalent colorings. Two elements are non equivalent means that both of them could not be transformed to each other by any element in the group. As I have mentioned in the answer (red,blue,red) and (red,red,blue) are correlated by the point 2 and 3 switch. Same for the pair (blue,blue,red) and (blue,red,blue). Thus are 8 – 2 = 6 nonequivalent colorings. If you are still unsure what nonequivalent means, please read chapter 14 of the book you gave, which includes all the defintions.