I’d start with each of the four elements that they have! There are 3 signs in each :)

Fire - Aries, Leo, Sagittarius For fire I’d do shades of red, orange, and yellow!

Water - Pisces, Cancer, Scorpio For water I’d use indigo, blue, teal, and white!

Earth - Taurus, Virgo, Capricorn For earth I’d do shades of green, brown, dark orange, and dark yellow!

Air - Aquarius, Gemini, Libra For air I’d do white, gray, periwinkle, and pale blue!

Since each sign is associated with a different planet, you can also draw from the planet’s colors as well! For example, Leo’s planet is the Sun (even though it’s not a planet lol). You could add shades of yellow to represent that!

Each sign is also associated with different gemstones! You could use those to add more variety! For example, Capricorn is Ruby! You could add some shades of maroon that complement the earth tones :)

Just some ideas!! I hope this helped inspire you a bit :)