There are three types of grunts: minor (orange), major (red), and spec-ops (black). However, they can all use the same weapons: plasma pistol, needler, plasma grenade, shade turret, and the fuel rod cannon. However, there are a few differences and weaknesses, as well as strengths, among the three.
Minor grunts are the weakest, yet most common throughout the game. Anything is good against them.
Major grunts are slightly tougher in terms of how many body shots it takes to kill one. They are also a little less common.
Spec-ops grunts are found near the end of the game, and are excellent grenade-throwers and use the fuel rod cannon FREQUENTLY. they also work together with spec-ops elites.
They all have one weakness: no matter what difficulty, it takes one pistol head shot to kill any grunt type.