i'm contemplating rattle canning my topper i use during hunting season to match the truck color... truck is in sig, and my topper is the lighter OE silver color. since there really isn't alot of surface area visible due to the full-length windows, and being a flat-top shell and sitting fairly high off the ground (translation: won't see top), i figure a less-than-perfect color match job would look better than than the color mismatch.
20 years ago, I rattle canned a complete '76 Celica w/Rustoleum "Sand" colored paint. It was three colors prior to this (the fenders were dif, and neither, and the hood matched one of the fenders). The new paint was a vast improvement.
I did the sides w/a can, and brushed the top and rockers (they had a wavy rubber coating on them) of a '89 MPV, and brushed a complete '90 Suburban (to much area for cans) w/XO Rust (from True Value) in Almond (tan).
On a red '80 Toyota PU, it had black primer fenders...they became Krylon red.
On all of them, a year later, it looked fine. I drive the Suburban now and people have to be told that it was painted w/a brush...btw, color selection really matters, it can be done w/white, sky-blue, grey, tan and a few others, but it has to be a light color.
If I was you, I'd buy a quart of WO rust for the top, and rattle the sides...OH yeah, I forgot to tell you...XO rust in a spray can, or a brush can is the exact same color : )
My '87 F-150 I bought in the mid nineties during high school came with a nice fiberglass topper but was white, truck was red. Did exactly what your suggesting- turned out rather well. You'll need about 8 cans, give or take.
I used to live by an automotive paint shop that could mix colors and put it in rattle cans. The shop by me now doesn't have that ability...but you might look around and get lucky.
Otherwise by the nicest paint you can get in a rattle can...ones that have a nice spray nozzle. If you put enough paint on it you can wet sand it flat and then buff it to match the shine on the rest of your truck.
Brandon - I recently replaced the front bumper on my daughter's '05 Lariat after someone backed into the truck. I ordered a new bumper from Rock Auto and ordered the paint HERE. The color was a perfect match (Arizona beige) but I went through a lot of paint getting a nice even coverage I was after (I applied 5 coats of color and 3 coats of clear). That said, I would order from them again. In fact, I bought some Tuxedo black for some touch up work on my own truck. The metallic for both colors was also spot on. They custom mix the paint and ship to your door. You can but rattle cans or just cans to apply with your own spray gear. I ordered rattle cans from them twice because I needed a little more of the Arizona beige and I had the paint in about a week. I also bought the clear top coat from them.