The paper stock for the interiors of books is determined by how much money is budgeted for the book. Paper weight is measured in pounds; colors are selected by the editor. There aren’t that many colors to choose from, in any case. The heavier the paper, the nicer it will feel. Our contract with our printer has a standard of, I think, 30 or 40 pounds for the paper. If the interior of the novel has photographs or art, you want a heavier paper stock, so that the drawings won’t reproduce too dark (the heavier the stock, the more ink it will absorb; the lighter the stock, the less ink). It all comes down to dollars and cents.
Due to to graphical natur of my book, and my need to make it have a quality feel to it, i want to use the heaviest paper i can.