I haven't used the tinted bed liner but I'm under the impression that it's without pigment, like a white or grey tone. You would use base coat to tint.
I'm with SuperB Bee, I'm not a fan of putting all this stuff on the underside or these cars for how they get used and driven today. I'm always supspect that there's something hiding underneath. If it's painted you can tell pretty quick what's going on. Not saying that's what's going on here, but what are you protecting the bottom side of the car from?
I will say this, I sure am glad my GTX was Ziebarted back in the day. The stuff is so nasty and dirty as I clean it off but that car looks like the day it left the assembly line. Greasy, nasty, YUCK!