There is an accepted principle that the prohibition of dyeing does not pertain to food. Therefore, one may add turmeric to a dish, even though it will color the food yellow. Similarly, one may dip bread in wine even though it will turn the bread red (SA 320:19). This is because the Torah prohibition is limited to long-lasting dyeing (like that of clothing or walls). Rabbinically, short-term coloring is prohibited as well. However, the short-term coloring of food is not included in this prohibition. This is because the primary concern with food is its taste, not its color; furthermore, people do not normally color most foods.
Some are stringent and avoid coloring drinks, as the change made to them is more obvious. Additionally, it is somewhat more common to color liquids than solids, since liquids are sometimes used for dyeing clothing and walls. Therefore, even though drinks are made for drinking, and not for their appearance, it is rabbinically forbidden to color them (Ben Ish Ĥai, Year 2, Pekudei 3-4). However, according to most poskim there is no prohibition of coloring even drinks. This is the halakha in practice (Darkhei Moshe 320:2). However, if possible, le-khatĥila it is preferable to defer to those who prohibit coloring drinks. Therefore, when mixing red and white wines, it is preferable not to pour the red into the white, as this will color the drink. Rather, one should pour the white wine into the red, because then the change in color is not obvious when the white wine becomes part of the red. Similarly, when preparing juice from concentrate, it is preferable to put the concentrate into the cup or pitcher first, and then add the water. This way the water is immediately blended with the concentrate, and the coloring is not obvious. Similarly, when preparing tea using liquid essence, it is preferable to put the essence in first and add the water to it (SHT 318:64-65; see above 10:8, which explains that one should pour the water from a kli sheni).