On most modern tanks (US for sure) any rings on the barrel are for identification and are not realted to "kills". Its very rare today for commanders to allow vehicles to have kill markings painted on, or even applied temoprarily. Jdeltaarrow hit it right on the head with the comment that crews do not want to draw any more attention to them selves than need be.
In all the units that I have worked with they have used Red, White, and Blue, for Platoon names. Each armor platoon has four vehicles and they are numbered 1-4. If the tank has four red rings then it is Red 4 (more than likely the Platoon SGT of 1st Platoon), if it had one blue ring than it would be the Platoon Leader of 3rd Platoon and so on. Most times any black rings would go with senior leadership such as Company Cdr, Executive Officer, BN S-3, BN Executive Officer and Battalion Commander.
Different units use different markings and call signs that are worked into or based off of the markings on their vehicles. I have used many different ones and have never used the same ones or styles for very long.