One of the thirty-nine "Melachot" – activities forbidden on Shabbat – is "Sobe’a," or "dyeing." During the construction of the Mishkan, the artisans would dye the animal skins used in building this structure. As the Mishkan’s construction serves as the basis for the Shabbat prohibitions, it is forbidden to dye materials on Shabbat.
The exception to this rule is food and beverages, regarding which Halacha states, "En Seb’ia Be’ochelin" ("There is no ‘dyeing’ with regard to foods") and "En Sebi’a Be’mashkin" ("There is no ‘dyeing’ with regard to beverages"). Thus, for example, it is entirely permissible to add a yellow spice to a food on Shabbat, despite the fact that one thereby transforms the food’s color. By the same token, one may make tea with a teabag or tea essence (in a manner that does not violate the prohibition of cooking), even though this results in changing the color of the water. Similarly, one may mix white wine with red wine, even though he thereby changes the color of the white wine. In all these cases, the prohibition of Sobe’a does not apply, since it is food or liquid that is being "dyed."
The question arises, however, as to whether this Halacha applies in a case where one specifically intends to change the food’s color. In the cases described above, the individual mixes the foods or liquids together in order to achieve the desire taste, such as the spice in his food or the tea essence in the hot water. Would the Halacha be different if one specifically intends to change the color of a food or beverage? Such a situation may arise on Friday night, when there is a Halachic preference to recite Kiddush specifically over red wine. If a person has only a very small amount of red wine, but a sufficient quantity of white wine, he may wish to pour the red wine into the white wine to make it red, so that he can recite Kiddush over red wine, in accordance with the Halacha. Is this permissible, or do we say that even regarding food and beverages, one may not change the color if his intention is specifically to change the color?
Rabbi Moshe Halevi (Israel, 1961-2001), in his work Menuhat Ahaba (vol. 3, 13:8), rules that it would indeed be forbidden to pour the red wine into the white wine with the specific intention to change the wine’s color to red. He therefore advises that in the case described, one should pour the white wine into the small amount of red wine. This way, he will not be considered as changing the color of the white wine to red. Hacham Ovadia Yosef, however, in his Hazon Ovadia, disagrees, and maintains that the prohibition of "Sobe’a" does not apply at all to foods and beverages, even in cases where one specifically intends to alter the color. In his view, it would be permissible to pour red wine into white wine to change its color.
As for the final Halacha, it would be preferable in such a case to pour the white wine into the red wine and thereby satisfy all opinions. Strictly speaking, however, one may certainly rely on Hacham Ovadia’s ruling and pour the red wine into the white wine.
Summary: Although dyeing materials is forbidden on Shabbat, one may mix or add spices to food and beverages even if this changes its color. However, if one has a small amount of red wine which he wishes to add to white wine so that it will become red, he should preferably pour the white wine into the red wine, though strictly speaking, it is permissible to pour the red wine into the white wine.
