Red Perks

Stopping Power Pro > 250 kills using stopping power to unlock pro
Stopping Power - 40% extra damage
Stopping Power Pro - extra vehicle damage

Stopping Power (SP) is generally the most used red perk. It is useful on all guns but the unsilenced WA2000 and magnum. On the FAL it allows for head shots within range. On all other guns it requires 1 less bullet to kill, either within range, out of range or both, with some obvious exceptions (snipers). SP Pro is pretty much useless as air support takes a long time to kill with bullets, launchers + cold blooded is the way to go.

Lightweight Pro > sprint 30 miles using lightweight to unlock pro
Lightweight > 7% speed increase (patched from 10%)
Lightweight Pro > quick aim down sights after sprinting

90% of the time you see a Lightweight (LW) class, it will be a tactical knifer. This is supposedly the reason for the 3% decrease. Ignoring knife classes, LW is useful but in limited situations. It is hard to justify using LW when it is up against SP and CB. That being said, it can be incredibly useful in objective games, capture the flag specifically. LW pro? eh, never really noticed much difference but hey, you might! Oh, and you will be sprinting for a long time to finish the challenges.

Cold Blooded Pro > Destroy 40 enemy kill streaks using cold blooded to unlock pro
Cold Blooded > undetectable by UAVs, thermal and air support
Cold Blooded Pro > crosshairs not red and no name visible when targeted

Cold Blooded is easily one of the most useful perks in the game for multiple reasons. It is the perfect weapon to take down air support, say goodbye to enemies getting more than 5 kills on the chopper gunner. Paired with a silencer and ninja you are completely invisible, so every time you come across an enemy it is a complete surprise to them. However compared to an unsilenced weapon with SP, the range and damage is considerably lower. CB pro, once again not all that helpful, but occasionally it will come in handy. Both the name and red cross hairs only come up at close range so it will only help if you are invisible i.e prone in a bush or something.

Danger Close Pro > 100 explosive kills using danger close to unlock pro
Danger Close > extra explosives damage
Danger Close Pro> extra air support damage

First of all, this is mainly a perk I use for fun. You know that annoying feeling when an explosive gets a hit marker but no kill? Pretty much gone. The downside being that unless you have an um… appropriate class, it will only give your nade a boost. Hmm, an assault rifle with grenade launcher attached, secondary RPG (thumper if not unlocked), any equipment, SoH (if you die often) or scavenger (unlimited explosives!). Green perk doesn't matter. Prepare to get yelled at, enemies don’t really like explosives much. Danger close pro only effects certain situations, such as the AT-4 against harriers.

Hardline > 40 hardline kill streaks to unlock pro
Hardline > kill streaks require one less kill
Hardline Pro > death streaks require one less kill

Pretty simple, each kill streak comes one kill earlier. To be honest I only find it worth using with a 2 (UAV), 3 (care package) and 4 (predator). Getting a UAV every second kill is pretty awesome. Hardline pro is once again useless, who wants death streaks, I swear they make you play worse.