Have a watch of this. Youtube: Computer vision processing tutorial.
If that makes vague sense and you can figure out which questions to ask to solve can length versus camera resolution whilst taking into account the lens focal length, then you've nailed it.
Otherwise, set your sights a bit lower and start breaking down the problem:
How do you detect a colour? What technologies, if any, do that, and can you buy a usable chip or module for your project, or even find a schematic online and build it yourself?
How do you detect "can length?" Electro-mechanical, computer vision... perhaps the can length can be measured by resistance?
How will you let only certain cans pass? How will the mechanism work and what powers it?
It is definitely possible to create your production line idea though. You can do it 