I really like this idea. It doesn't have to be a "yellow card", per se, but the idea of an official warning with potential repercussions - as opposed to an instant ejection for what may have very well been an incidental, unintentional, high hit - could very well be an effective way to handle the new targeting rule in order to prevent ejections from getting out of hand.
We actually already have this, or at least a precedent for this - in a way - in college football already: the sideline warning. The referee officially warns the team of its sideline interference and the team knows that the next time it happens there will be a penalty.
In regards to the targeting rule, the referee gives an official warning to the player who commits a foul deemed by the official(s) to be in violation of the targeting rule and the team - both the coaches and players - know that if that player commits that foul again within a certain amount of time then that player will be ejected. The initial hit, or foul, is still a 15 yard penalty. The review process - after the game - gives the officials and/or conferences the ability to either upgrade the severity of the foul to include loss of playing time for that player if the foul is deemed to be egregious or, if it is deemed that the official(s) made a mistake, to even remove the "yellow card" warning from the player.
I would prefer to treat the warning, or "yellow card", process a little more like soccer, though, in regards to timing or aggregation of the warnings. Rather than have the warning stick with the player for the rest of the season, perhaps it only sticks with them through the next game - as a yellow card usually does in soccer. You could also add a "repeat violator" clause - to prevent the player(s) from being able to commit the foul once every other game - that makes any player's violation of the targeting rule an automatic ejection after their third or fourth warning of the season. This would prevent players from being able to be reckless until they get a warning, then take it easy for a game until the warning "expires", and then resume their reckless play. This also would prevent, for example, the situation where a player who made a mistake and committed this foul in the first or second game of the season ends up missing 58 minutes of their bowl game because their adrenaline got the best of them - or the official(s) "saw" the play incorrectly - and their second violation comes 9 or 10 games after the first one.
I also like the idea of some sort of fine or penalty for schools and/or coaches whose players accumulate too many warnings over a season, clearly putting the responsibility for proper coaching - along with the penalties for failing to do so - where it belongs: with the schools and coaches.