One of the thirty-nine categories of forbidden activity on Shabbat is "Tzovei'a," or "dyeing." Halacha forbids coloring a material on Shabbat, just as Benei Yisrael dyed animal skins as part of the construction of the Mishkan.
The question thus arises as to whether on Shabbat one may wear auto-tinted glasses, which become automatically tinted when exposed to sunlight. When a person walks with these glasses into the sun, the lenses change color, and as such we might consider forbidding wearing these glasses on Shabbat.
In truth, however, the authorities allow wearing such glasses on Shabbat, because this form of "coloring" differs significantly from the dyeing that was done as part of the construction of the Mishkan. Benei Yisrael dyed the animal skins by placing them in dyes; here, the sunlight creates a chemical reaction in the lens that triggers a change in color. Since this process does not at all resemble the process of dyeing animal skins, "coloring" the lenses of these sunglasses does not fall under the category of Tzovei'a and is therefore permissible. (See Halichot Olam, Helek 4, page 282)
It is customary for a Mohel (person who performs circumcisions) to dip a gauze pad into wine at a Berit when the Rabbi reads the verse, "Va'omar Lach Be'damayich Chayi." The Mohel then places this gauze pad in the infant's mouth for him to drink a drop of the wine. Would this be forbidden on Shabbat, given that the gauze is colored by the wine?
Halacha allows coloring a material if it is done "Derech Lichluch," meaning, if it soils, rather than enhances, the material. Therefore, it would be permissible for a Mohel to dip the gauze in wine, since he thereby soils the gauze, rather than "coloring" it in the sense of beautification. (See Yalkut Yoseph, helek 4, page 346.)
May one apply a bandage to a bloody wound on Shabbat, or would this constitute Tzovei'a by virtue of the "coloring" of the gauze by the blood?
Preferably, one should wait for the wound to stop bleeding before applying a bandage on Shabbat. If, however, the bleeding does not stop one may certainly apply a bandage despite the coloring effect of the blood on the gauze. (Orach Hayim, siman 328:4. Also see Menuhat Ahava, Helek 3, page 12.)
Summary: One may wear on Shabbat sunglasses that automatically become tinted in the sun. A Mohel may observe the customary practice to dip gauze into wine during a Berit even on Shabbat. One who has a bloody wound should preferably wait for the bleeding to stop before applying a bandage; if the bleeding does not stop, however, one may place the bandage even while the wound still bleeds.
