Speaker Wire for subs?

hello everyone, i have a ? on size of speaker wire.  Running straight from the sub, through the box (no terminals) what guage wire should i use, my amp can handle as low as 8g wire.  Should i use 8g wire for 500w or is 10 ok.  also if 8 is good, what do u think of using red power and black ground as the 8g wire since i have so much of it.

Thanks