From the Plastic Classic forum, buried among all the other treasures, is this expose on epoxy fillers and microballoons. Tim writes:

the color of Tim’s System III micro balloons

the color of Tim’s System III micro balloons

I don’t generally use the fillers sold by West System, partly because I have found that I prefer other brands better, and partly because of available container size and cost. The West System version of microballoons is #407 Low Density Filler. Don’t let the name “low density” fool you; this doesn’t somehow imply weakness or other unsuitability. However, it does mean that one tends to get tiny pinholes in the sanded, faired product, and these pinholes must be filled with either several coats of unthickened epoxy resin, and/or with a finer fairing compound. All faired areas should be sealed with at least one coat of thin epoxy before painting.

The phenolic microballoons that I use are System Three. They tend to be reddish-brown in color, tending towards a bit of purple tint, but essentially they are the color that you see in the sanded patches on the boat. When soaked with resin, they darken somewhat.

All microballoons are reddish-brownish-purplish. Microballoons make a great fairing compound because they sand fairly easily, are strong, and lightweight. They work because, as the name implies, they are actually little tiny hollow spheres that absorb the resin inside the sphere, thickening the mixture. By the same token, atmospheric conditions affect how the microballoons look and feel in their dry state. Very dry conditions tend to make them powdery, while damper conditions often allow them to be soft and almost more sticky on their own. (This doesn’t apply to West #407, which is always very dusty.)

There are other types of fairing fillers that are lighter colors; these don’t contain microballoons. If you’re using a West System powder filler that is white or off-white, it’s likely to be #404 High Density Filler, which is actually an adhesive filler. This isn’t the choice for bulk fairing and is designed for other uses, such as bonding and fillets. West System #410 Microlight filler is very lightweight and easy to sand, but it softens on surfaces such as decks or dark surfaces that are exposed to high heat, and should not be used as a general rule. This filler is tan.

Most of these fillers are only semi-thixotropic, and require the addition of some amount of cabosil, or colloidal (fumed) silica, in order to become a thick, non-sagging compound of any practical use. Cabosil (West System #406) is an adhesive filler on its own, and good for making thicker glueing pastes. Epoxy thickened with cabosil alone is extremely hard to sand, and is not suitable for fillets, or fairing, or many other common uses. But the cabosil is an important modifier for any microballoon mix.