I agree with Matt. There are many variables that could affect color. If you paint it together, you still have the problem of the area between the bed and cab, plus there isn't a good result when trying to spray the fenders on the truck either.

Try to keep the paint well mixed each time, with the same amount of reducer, and have the shop conditions as close as possible. If you don't plan on perfection, you will be a little more accepting of the results.

As a minimum, I would suggest you shoot the outside of the cab with the doors and hood installed. Get it totally prepped in primer. Shoot the inside first, then assemble the parts, and carefully mask the jams off, before you spray the outside.

Do you have a final color in mind? That can make a difference in how difficult the match is.