This thread thickness is the same as Danville 210 Flat Waxed Nylon except it’s twisted rather than lying flat on the hook. They both are capable of high tension making them ideal for tying fresh and saltwater flies. 

WHY DIFFERENT COLORS?  I usually tie different colors to identify specific flies, such as red heads for heavily weighted, orange heads for medium weights and yellow for shallow water nymphs. I also like dry flies with different color heads. Another reason is to match colors close to the body material when using floss, silk, or any other material that under-color bleeds through when wet.