I've had Rhino Liner in two trucks since 1996. Both were sprayed in black, and within a year, they both turned grey. The sun's ultra violet rays are the culprit, and unless you keep it covered (tonno or shell), the black will always turn grey.
Not sure about the Line-X...
So until they come up with a spray that maintains its color, I'll probably stay with a drop-in.
Chris in NC