From the manufacturer:
Field Drawing Books contain 50 sheets of cream colored 400 Series Drawing paper, ideal for classroom or field work. Black Field Drawing Books contain 50 sheets of black drawing paper, excellent for colored pencils or gel pens. The books are encased between durable black hard covers and the sturdy wire construction allows the books to lie flat. CREAM: 80 lb. (24" x 36" x 500 sheets) 130 g/m2 BLACK: 98 lb. (24" x 36" x 500 sheets) 160 g/m2