Shabbos 73a: Dyeing is one of the 39 categories of work forbidden on Shabbos.
Yerushalmi Shabbos 51a: One who dyes red lips red is liable for working on Shabbos.
Shulchan Aruch Orach Chaim 320:20: One who eats strawberries or other colorful fruits must be careful not to touch his clothing or the tablecloth with colored hands, because this would be dyeing.
Magen Avraham: This is all the more true if the garment is red, because then he would be improving its color with the red juice on his hands.
שבת עג ע”א והצובעו.
ירושלמי שבת נא ע”א המאדם אודם בשפה חייב. קרבן העדה: פירוש אפילו בשפה שהיא אדומה אלא שמוסיף בה אדמימות חייב משום צובע.
שו”ע או”ח ש”כ ס”כ: יש מי שאומר שהאוכל תותים או שאר פירות הצבועים צריך ליזהר שלא יגע בידיו צבועות בבגדיו או במפה משום צובע.
מגן אברהם סקכ”ה: ונ”ל דבגד אדום כ”ש דאסור לקנחו דמתקן הוא.
Rabbi Moshe Chaim Lau described how his grandfather, Rabbi Yitzchak Yedidya Frankel, went to obtain semichah from Rav Shlomo Dovid Kahane zt”l of Warsaw. R’ Shlomo Dovid was famed for his expertise in finding ways to free agunos and permit them to remarry. Indeed, he succeeded in permitting thousands of such unfortunate widows after World War I. During World War II, he immigrated to Eretz Yisroel. Many more agunos came to his door after the Holocaust.
Every young rav in Poland dreamed of receiving semichah from Rav Shlomo Dovid, for this accomplishment guaranteed a coveted position in a prominent community. Rav Shlomo Dovid regularly delivered a shiur on Shabbos from three in the morning until Shacharis. When the yungeleit would arrive in the driving snow, he would smile widely and offer each one a cup of steaming tea. Generally, some ten yungeleit gathered around his table for the shiur. Now and then, he would address one and ask him a challenging question. Every once in a while, he would tell the young man to come back on Sunday to receive his writ of semichah. Often he asked the yungeleit to come back for a few Shabbosos to be tested again.
When Harav Frankel came to attend the Shabbos morning shiur, the subject was hilchos Shabbos. Harav Kahane addressed an outstanding young man and asked him what to do if someone is bleeding heavily on Shabbos. Should he use a red cloth or a white cloth to stanch the bleeding?
The spontaneous answer, coming from someone who doesn’t know the halacha, is to use the red cloth, because if he uses the white cloth, he will color it red with the blood. Someone familiar with siman 320 in the Shulchan Aruch, however, knows that the Magen Avraham ruled it is better to use the white cloth, explaining that the blood dirties it, which is not considered coloring. On the contrary, if the red cloth is used, the blood colors it a deeper red. The Eliyah Rabbah has a different opinion. Because he was unaware of this information, the young man simply smiled and said that if such an unusual incident would happen, he would hurry to open his sefer and find out what to do. Practically speaking, however, such a case would never come up.
Rabbi Frankel said that he would never forget Rav Kahane’s reaction to this answer. He froze in place and turned ghostly pale. Banging his hand on the table, he screamed out, “A Jew’s blood is flowing, and you have the leisure to open a sefer and study it?” Needless to say, that brilliant young man never got his semichah.
Rabbi Frankel saw this incident as a seminal lesson for every rav. They must know the halacha before the case comes up. They must be able to act quickly and decisively, and above all with care and empathy for another Jew’s suffering.
Source: Hamodia, Inyan, Vayechi 5781, p. 20