Why tooth powders are often colored red (1873)

“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.