It's elemental.
Each sign and planet is associated with an element
Aries, fire
Taurus, earth
Gemini, air
Cancer, water
Leo, fire
Virgo, earth
Libra, air
Scorpio, water
Sagittarius, fire
Capricorn, earth
Aquarius, air
Pisces, water
The fire signs are represented by reds, oranges, and possibly yellow.
the earth signs are represented by greens, browns, and other earth tones
air signs can be yellow, blue, grey, and whites (representing clouds)
water signs can be dark blues, whites, possibly greens.
As for the planets, well thats a different story
moon-earths sattelite-white/silver
mercury-the messanger-yellow
venus-the lover-green
mars-the warrior-red
jupiter-the king of all gods-purple
saturn-the lord of time-blacks and dark earth colors
uranus-the rebel-light and dark blue
neptune-lord of the seas-dark blues and sea-green
pluto-lord of the underworld-blacks
certain signs associated with certain planets have a very specific color that do seem to click easily but not always:
Cancer-moon-silver/greys
Leo-sun-yellow/oranges
scorpio-mars and pluto-red and black
Aquarius-Saturn and Uranus-usually blue but Aquarius is the rainbow of the airsigns and thus can use all colors.
the combinations are infinite so don't worry if you don't notice it right away.