From reading what you wrote I am assuming it will be
exposed so that others see it. If that is the case
I would go to the dealer and see if you can get some
spray paint that is an exact match to your paint code.
I recently did the inside of the bed on my Tundra. It was
going to be covered by the bed
liner so I wasn't too
picky with the color match. I bought some self etching
paint at AutoZone along with a spray can of clear coat.
What year is the truck? If it's an older truck and its
condition isn't that great then maybe you can pull this
off, but either way I think it's going to show that you've
taped and painted a section of the truck. If what I read
is wrong and the paint job is going to be concealed by
the bed
liner and rails then have at it and paint to your hearts
content. Hell, paint it pink if nobody is going to see it! Good
luck.
John