There are all kinds of fillers. These are the ones that I'm familiar with.

Glass bead micro balloons. This is a white powder and is what most people are familar with. This is the standard filler and makes for a very light filler. It should not be used for any structural purposes.

Phenolic micro balloons. This is made from milled phenolic (bakelite) and is a very dark brown. Same comments as above apply. I often use a combination of these two micro balloons to achieve the colour match I'm looking for.

Cabosil (trade name). This is made from silicate and is a thicksetrope. Add only a bit of this to your epoxy and it will become very viscous. It is typically not used as a filler for fillets etc. Cabosil is very difficult to sand (but not impossible). It makes a better structural filler for eg. scarphing sheets of ply wood together.

Saw dust and Flour (as in all purpose, whole wheat) work as well and fall more into the category of macro ballons (my terminology). These would work like either glass or phenolic micro only it would soak up more epoxy and make for a slightly heavier mix and be a little harder to sand.

I'm no expert and would like to hear from other people.