We know that the color red represents Din. We see this with Esav, who was red. In fact, he was a שופך דמים, a murderer, another aspect of Din. What is the name of his nation? אדום, which means red.
R' Moshe Shapiro pointed out that throughout the world the color red means “stop” and red is midas ha’din, which expresses brakes and limits. This explains why signs that tell us to stop are red, including the red light on a traffic light and the stop sign.
Breishis 25:25.
Breishis 25:25, Rashi. Esav descends from Yitzchak, the one who possesses the trait of Din. Also, he refused the Torah because of לא תרצח, Din. We also see his connection to death (Din) from his Malach who is also the מלאך המות.
We are taught that Esav married two women from חתי (Breishis 26:34). חתי is related to the term חת, fear (see Yirmiya 30:10), which is rooted in Din.
A comical story is told of the Satmar Rebbe who was traveling in a car when the red traffic lights were something new. Upon his driver stopping at the red light, the Satmar Rebbe inquired why he did so since he wasn’t aware of such a thing. After the driver explained it to him, the Satmar Rebbe said this explains ויאמר אליו אדום לא תעבר בי פן בחרב אצא לקראתך, when it is a red light, don’t pass because it may be precarious.
The color of blood is red. This is alluded to in the fact that the root of the word אדום, red, is דם, blood. He also pointed out that on the other hand, green is the accepted color for “go” and is the color of midas ha’rachamim, expressing continuation and future. Also, blood is red and people get squeamish from blood.
Bamidbar 20:18.
