One of the thirty-nine prohibited Melachot on Shabbat is "Seviah"-dyeing. When the dyeing is permanent it is an Isur D’oraita (Torah prohibition) and when it is temporary, an Isur D’rabanan (Rabbinic Prohibition). The Menuhat Ahaba (3:13) discusses the case of a toilet bowl cleaning device which releases a colored detergent. Every flush causes the water in the toilet bowl to become colored. Does this present a problem of Seviah on Shabbat? He first establishes that at worst, it would be an Isur D’rabanan, as the color remains only temporarily.
He presents several reason to permit their use. First, the dyeing is not apparent, as the water was already blue from before Shabbat. Each flush merely continues the previous state and does not change the pre-existing color. Secondly, one does not have Kavana-intent to dye the water. Moreover, the dyeing happens indirectly as a result of flushing water, which renders the action a "Gerama." Based on all these reasons, he permits flushing a toilet which has such a device already affixed from before Shabbat.
Shulhan Aruch writes that one must "be careful" not to wipe off residue of colored berries from one’s hands or face with a towel. Such an action dyes the cloth and constitutes a prohibition of "Seviah." Hacham Ovadia rules in accordance with Maran, even though his source is the lone opinion of the Yere’im. It is better to first rinse his hands or face with water to remove the residue and then wipe with the towel. The Mishna Berura Halacha (Rav Yisrael Meir Kagan of Radin, 1839-1933) concedes that if a person has no choice, he may wipe his face directly. Similarly, the Hida (Rav Haim Yosef David Azulai, 1724-1807) points out that the language of Maran is ‘Yesh L’Hizaher"-one must be careful-which means that it is good to be strict, but it is not an absolute requirement of the Halacha. This is also the conclusion of Menuhat Ahaba, who also explains that there is no concern that his face and hands became stained by the fruit, as there is no issue, D’oraita or D’rabanan, of "Seviah" on the body.
There is no problem to flush a toilet containing a bowl cleaner that colors the water.
One should rinse his face and hands to remove colorful fruit residue before wiping them on a towel. If there is no choice, he can wipe the residue off directly.