i have extra 8 gauge power cable (its not wrapped red like my other one, this one is wrapped in black)

but i need to make my ground connection longer, so it is okay to use that extra cable i have?
cuz i figured its all the same cables, just the colours denote the uses

so basically, is it safe to use my extra power calbe for grounding?

 

wire = wire, you're correct.

That cable is fine.

 

just keep in mind that the ground should be a short commute from the connection to the physical ground...

just keep it as short as possible (without major complications) with that 8guage and you'll be fine