There are many ways to handle pigments prior to tempering--dry, wetted and mulled; dry, wetted and mixed with a palette knife; in dispersion; or kept in distilled water. I think I'm coming around to the last example.
Before my store bought pigments arrived, I mulled North Carolina pigment prior to mixing into a tempered paint. What's important though is that mulling is only partly used for and useful for grinding pigment. Better grinding is probably accomplished with a mortar and pestle. Where mulling really shines is for separating particles simply stuck together and more importantly surrounding each tiny particle with water.
Instead of mulling, particles can sometimes easily be mixed well with water using only a palette knife. This is my most often used technique. A palette knife is quite sensitive to particle size. I can see, hear, and feel grit with a knife against my porcelain palette.
I have a few bottles of dispersions. They're okay but the pigment particles are held in suspension with the help of additives. I don't know what those extra ingredients are and don't really detect anything unique about them. One problem I find with dispersions is that when I open the cap, there is always some dried pigment that falls out and lands on my palette. I hate that. The other thing is that getting just the smallest drop I can is often too big. So, there is convenience and inconvenience.
And now, my the final technique. Dry pigment is either mulled or mixed with a palette knife, placed into a jar, and covered with distilled water. A rather hard to discern example is below. My apologies, as this is German Vine Black and really doesn't show very well. There's about a 5mm layer of pigment settled in nicely. I also used a bit of isopropyl alcohol as a wetting agent. By the way, this jar came from Penzey's Spices. I buy my herbs and spices in bags and keep cooking amounts in these four ounce jars.
When I want to use German Vine Black, I open the jar, tip it to the side, and scoop out just what I need. It's already mixed, wetted, and the perfect amount. Although this was only mixed with a palette knife, it should probably have been mulled as I can see granulation in my tempera. That's cool if that's what I want, but I am leaning towards smooth pigment mixes so I will probably mull this batch. And, I will probably mull all pigments as I jar them up.
My palette knife is an okay tool for scooping pigment but I want something still smaller. I envision making my own tiny trowels for scooping and mixing.