Shabbos – Colouring, Writing and Drawing with Foods
Although there is a prohibition to dye on Shabbos, dyeing foods is not prohibited. The reason for this is that the process of dyeing must be performed with the same purpose as dyeing was performed for the construction of the Mishkan, the Tabernacle. [this is true in regards to all the Melachot that are prohibited on Shabbos] The Mishkan required dyeing of cloth, i.e. to make a permanent decorative mark for a lasting display. Adding colour to foods does not match this same purpose.
However, as the Peri Magadim advises [noted by the Mishneh Berurah] one should not add colour to water for example, if the purpose is for display. Perhaps this might be a concern when a “display centre piece” is being prepared for the table, even though it is only a temporary display. [I include a range of references so that the intensity of discussion can be appreciated. - Bais Yosef 320, Darchei Moshe 2, Pri Megadim M.Z. 318:15, Eishel Avraham 320:22, Chacham Tzvi 92, Rav Poalim 3:11, Kaf Ha’chaim 113, Yalkut Yosef 320:9, Yesodo Yeshurin 4:pages 68-69. Refer to Zochor V’Shomer page 7, The Shabbos Home 2:page 338, Ohr Yisroel 8:Page 92. Refer to Yalkut Yosef 320:footnote 13, Yabea Omer O.C. 2:20, 10:55:pages 157-158, Yechaveh Da’as 6:23, Shulchan Aruch Hamikutzar 62:footnote 54]
So, there is no problem in adding milk to coffee or tea, diluting cordial or combining mustard with mayonnaise. Similarly, there is no issue using those toilets that have a disinfectant/perfume that also colours the water, since the colour is not for aesthetic purposes.
Others say that coloring drinks would be problematic. [Elef Lecha Shlomo 136, Ben Ish Chai Pekudei 2:3, Kaf Ha’chaim ibid]
Writing on Foods
Although writing and drawing is prohibited on Shabbos and Yom Tov, simple decorative swirls of various pastes and liquids, and random sprinkling of colourful powders may be applied on Shabbos. [Harav Yisroel Belsky; OU document S-18]
Breaking Writing on Food
It is also forbidden to erase writing and drawings on Shabbos and YomTov. The Mishnah Berurah rules that markings which are a separate component to the food, such as piped icing, may not be erased but the food with the markings may be eaten even though this erases the markings. [O.C. 340:17, see Chazzon Ish 61:1] However, if the drawings or lettering is embossed or engraved in the food, it may be broken or cut without concern. [Magen Avraham 6, Mishnah Berurah 15. The Aruch Ha’shulchan 23 suggests we ought to be concerned about this]