Among other things, blue is associated with water, which is in turn associated with life. In Kabbalah, blue is one of the colours of Chessed, lovingkindness. Red is its polar opposite, the colour of blood and fire, representing Gevurah, judgement and severity. Combining red and blue gives purple, the balance between them, Tiferet, seat of truth and beauty. On a mystical level, the purple argaman (ארגמן) also represents the chief angels Uriel (אוריאל), Raphael (רפאל), Gabriel (גבריאל), Michael (מיכאל), and Nuriel (נוריאל).
...the colours of the three key Sefirot of Chessed, Gevurah, and Tiferet appear to be blue, red, and purple, respectively.
...The Ramak then says that Tiferet must be, as expected, a blend of Chessed and Gevurah, so a mix of white and red, or blue and red. This might suggest pink or purple, and the Ramak notes the connection to argaman. He also suggests the possibility that it is yellow, like the yolk of an egg. Alternatively, he cites Rabbi Shimon bar Yochai as teaching that Tiferet is green (as the Zohar says, for instance at III, 257a, Ra’aya Mehemna). When we look at the colour spectrum, green actually makes sense. The reds are on the left end (left being Gevurah), and the blues are on the right end (right being Chessed). In the middle lies green, just as Tiferet lies in the middle between Chessed and Gevurah.