The Halachah of Colors
Halachah speaks of four basic colors: shachor (black); adom (red); lavan (white); and yarok—which can refer to shades from blue to yellow and any combination of them in various shades of green. There are other colors that are used in the Torah, some which are obsolete today, but those colors are generally named for a specific item or coloring agent of that shade. One example is the blue (or purple) color of a dye called techeles, which is made from the blood of the chilazon (a type of marine creature).
There are many halachos that reference color. The Shulchan Aruch calls red out as a flashy color and many women do not wear garments of that color for reasons of tznius (modesty). The definition of the color red is important when it comes to blood—for identifying nidah (menstrual blood) and blood spots in eggs—but poskim state that our knowledge of the color red is not comprehensive enough to identify it precisely when it comes to niddah, so red is classified as a range of reddish hues in these cases.
Diagnosing tzoraas also depends on the special knowledge and identification of certain colors. When it comes to the kosher status of animals, discoloration of organs in various shades identifies conditions that classify them as treifos (non-kosher).
The earliest z’man (designated time) for donning tzitzis (or a talis for davening) is also related to color recognition: the sun’s light on the horizon must illuminate to the extent mi’she’yakir (that enables one to recognize) the difference between the shades of lavan and techeles. Techeles is itself essential to the mitzvah of tzitzis (though the tradition for creating this exact dye has been lost for many generations).