Because in Truth there's only four colors.
In the Holy T. Zohar and Zohar it's brought
- White and red are Rachamei and Din
It even gives concrete examples white stones are kindness, red din.
- Green/yarok and black, the Tzaddik Center Pillar and Malkhut
Also mentions this for people's skin colors and maybe hair.
So for example Yarok/"green" skin people (i.e. olive yellow tan?) are prone and apt to attach to the Tzaddik. Maybe this applies to the hair that's blondish dirty blond brownish as that would be yarok? Don't remember
And then Tekhelet includes all the colors, lashon Kol.. that might be closest to "blue" so to speak well look at the sea and the sky. That's Tekhelet. One we call Blue (blue means cyan, light blue by the Classicists) one Indigo in the foreign concept ROYGBIV. Likewise in some foreign languages like Russian you have two different words for blue, Sinii (cognate cyan?) light blue Goluboy dark blue, it's like they see two colors of blue and we English speakers see one! Some say language shapes thought so .. when in doubt actually l'chatchila bring it back to the Torah and Chazal and the True Tzaddikkim and their advice and b"ehy all the "supernal colors will shine for you" (a phrase in the Z.) and all should be clear for us.
Update: here are search results where you can read more about Rashbi and friends' holy teachings on the four primary colours (and they correspond to YKWK, many other things that are four, so discern) which are the root of all the other (many, infinite) "giwwunim"/colors.
The terms you want are
חִוָּור סוּמָק יָרוֹק אוּכָּם
Chiwwar Su-mak Ya-rok U-kam
"White Red Yellow-green-tan-whatever-lightish Black"
Remember this is also Supernal things first up above and not just physical light down below.