The most common paper used for coloring books is 60lb uncoated text, which is similar to standard printer paper. A slightly thicker option is 70lb uncoated text. Both are great for colored pencils or crayons but Sharpies will likely bleed through. The thickest uncoated text paper is 100lb text. It's not typically used for coloring books but it could be a better option if you're looking to reduce Sharpie bleed through.