First off, we need to understand the difference between a solution and a suspension. When you dissolve something in a solvent like water, alcohol, or oil, the substance separates into individual molecules floating around in the solvent. A solution may be colored, but it is transparent, like apple juice, grape juice, or red wine, because the molecules are far too small to be seen with the naked eye. A solution does not settle out over time. A suspension, on the other hand, consists of particles much larger than molecules floating around in the solvent. These particles may be fine powder to our eyes, but they are still much larger than molecules. Suspensions are opaque, like milk, orange juice, and blood.

This brings us to the distinction between dyes and pigments. Dyes form solutions. When you dye cloth, you dissolve the dye in a solvent, usually water, and then soak the cloth in the dye solution. The dye penetrates the textile fibers and binds to them chemically. The result is that the color becomes an integral part of the textile fiber. It will not flake or fall off, and will not wash out easily.

Pigments, on the other hand, are mixed with a binder that sticks only to the surface like paint. If you dye a canvas, the color becomes part of the cloth. But if you paint a canvas, the paint sits on top. You can add additional layers of paint, and you can scrape it off if you wish.

So what do you want to use to color soap, a dye or a pigment? Both may be used. If you want to color transparent soap, you must use a dye. But for opaque soaps, pigments are much more useful. Since dyes are soluble, they bleed into one another. Pigments stay put, so it is possible to swirl and layer soaps of different colors while maintaining sharp boundaries between the colors.

Compared to dyeing or painting cloth, soap presents some additional challenges. Dyes are only useful for cloth if they bind to the textile fibers. Dye that binds to skin, though, would not be desirable for soap. Pigments require a binder to stick to a surface, and since we don't add binders to soap, pigments are fine to use in all opaque soaps. Additionally, dyes and pigments used in cold process soap must survive the harsh alkalinity of the lye solution. Colorants do not have this problem when added to hot process soap after the cook or to melt and pour soap.

One more consideration is that whether a colorant is a dye or a pigment depends on what solvent you are using. Water is the most common solvent for dyeing cloth, and consequently most plant dyes are soluble in water. Since raw soap contains more oil than water, plant colorants that would be considered dyes for dyeing cloth are likely to be pigments in soap. That is, they dissolve in water but not in oil. The majority of plant colorants, then, will be most appropriate for opaque soap.

A final consideration is that plant colorants can react with lye, just as oils do. Some molecules may decompose, while others simply change colors. An extreme example of this is red cabbage. If you squeeze the juice out of a red cabbage, the juice is deep red or purple, as you might expect. But if you add some lye solution to it dropwise, the color will morph from purple to blue to green to yellow. Raw soap is at the far end of this range, so red cabbage makes soap yellow. In fact, red cabbage juice may be used as an alternative to phenolphthalein for testing soap alkalinity. A few drops placed on a bar of soap will turn yellow or green if the soap is overly alkaline, but blue when the lye has been consumed.

Many plant dyes used to dye cloth make excellent soap pigments. Indigo is the dye most familiar as the color of blue jeans. It is quite stable in soap, and may be added to the lye solution. Madder root dyes cloth red, but colors soap pink. Like indigo, it may be added to the lye solution.

Some plant dyes that would be dissolved in water for dying cloth are actually more soluble in oil. They can be infused into oil for making soap by simply soaking the dye in liquid oil until the oil takes on the color. Annatto seeds produce an orange color on wool, but yellow to gold in soap. Alkanet root colors soap blue or purple, turmeric colors it orange, nettles green, and saffron yellow.

While minerals like oxides, micas, and ultramarines dominate opaque soap colorants, and FDA-approved D&C dyes are popular in melt and pour soaps, plant dyes traditionally used for textiles can make excellent soap colorants. When experimenting with new materials, start with small batches to find out how the new colors behave in the alkalinity of raw soap. Try adding the colorant to lye, oil, or soap batter to see if one works better than the other. If a color is unsatisfactory in cold process soap, try it in hot process soap after the cook. And finally, monitor the color as the soap cures to see what changes may occur. Plant colorants open up a wide spectrum of options for soap makers seeking to add beauty and label appeal to their products.