Camaraderie mentions an excellent point regarding paint. Whether for auto or boat paint varies from can to can, even in the same batch. Ask any body man who has found himself just a little short on paint when doing a car. He pops open another quart, from the same batch of paint, sprays and it doesn't match. He then spends half a day blending it in. That job takes a lot of skill and experience.
Don't tell me about "factory" paint, batch run numbers, etc... It happens and the reason doesn't matter. And with the price paid for the coatings were talking about the fix can easily put one over budget.
The solution is to buy more paint than needed, as Cam suggested, and mix it all together prior to application. After all of the paint has been mixed you will have one color. Now it can be returned to it's original containers. The excess is best stored in the smallest container you can find. If you've got a quart left in a gallon can, transfer it to a new quart can. Store it where temperature extremes are minimized and it doesn't hurt to shake it up good once a year.
For those with no experience with this I would emphasize that the color variations will not be obvious to the eye; in the cans, during application, or even indoors in the painting booth (esp. not under flourescent light), but only after drying and in sunlight. It will be particularly noticeable at twilight when the sun's rays are more horizontal.
Cruising your local mall can illustrate the point. Look at the front fender/door jamb line on the parked cars. It won't take you long to find one that's "just off", imagine if it were your boat!