something that will disperse paint and have that "pop" sound, adding realism, but still be safe for use... like if some accidently held on the the grenade, it wouldn't injure the person...
I don't see how you could get both enough force to spray paint everywhere and not enough force to *ever* hurt someone. In my mind, baking soda+vinegar couldn't get enough force for a decent pop; You'd have to go with some sort of improvised explosive Which you probably shouldn't do
However, I think you could get a healthy amount of safety if rather than grenades you made remote mines- take a non flamable base, attach a very small explosive (like a small firecracker) to the center, hook it up to some sort of electric trigger, then put a paint balloon on top. When someone passes by, you push the trigger, setting of the explosive and spraying paint everywhere. The base would make sure the ground isn't effected, and the paint on top would probably muffle the explosion to the point it wouldn't hurt anyone. Even if the explosion was potentially harmful, you can control exactly when and where it goes off, rather than a grenade that could easily go some place you don't want it to go.
Ultimately, I think you should heavily consider how no such item exists. Presumably, there is a big demand for realistic paint grenades; and yet none exist. Why do you think no one makes them? Because no one wants to get the crap sued out of them.