“To remove the defect of being apt to accumulate between the folds of the gums and in the cracks and interstices of the teeth, charged against the white powders by those who use them carelessly, a reddish or flesh-coloured tinge is commonly given them by the addition of a little rouge, red coral, rose-pink, Armenian bole, or other harmless colouring substance. In this way, any portion that may remain unrinsed off is rendered less conspicuous.”
Instructions and Cautions Respecting the Selection and Use of Perfumes, Cosmetics and Other Toilet Articles, by Arnold James Cooley, 1873, p. 512
Red-colored tooth powders show up much earlier, throughout the 1700s and 1800s, but this is the only primary source I’ve been able to find that explains why.
“Take Rose-water, Syrup of Violets, clarified Honey, and Plantain-water, of each half an ounce; Spirit of Vitriol one ounce; mix them all together; rub the teeth with a linen rag moistened in this Liquor, and then rince the mouth with Rose and Plantain Water, of each equal parts.”
The Toilet of Flora, 1772
Fresh-roasted coffee (in fine powder) …. 1 1/2 ounce
Chocolate (do.) …. 1 1/2 ounce
White sugar (do.) …. 1 1/2 ounce
Vanilla (do.) …. 1 ounce
Charcoal (recent; do.) …. 1 ounce
Mucilage of tragacanth (to mix) …. q.s.
The preceding, sucked ad libitum, are used to sweeten and perfume the breath; the last also acts by chemically deodorizing it. They are great favourites in the fashionable world among smokers.
Instructions and Cautions Respecting the Selection and Use of Perfumes, Cosmetics and Other Toilet Articles by Arnold J. Cooley, 1873, p. 551