Hair coloring question?

wen i was younger i had natrually light blonde hair, but i began to color it just out of curiousity and to get different blonde shades.but as ive gotten older my roots have gotten darker, theyre still blonde just a dark blonde, anywase i want to color my hair dark blonde to match the roots and to avoid having bad roots, but everytime i try to take my hair a darker color it comes out green or like a gray color, ive tried using warm colors but the same thing happens, i can not afford a proffessional, so does anyone know anyway to get my hair to a darker blonde without messing it up, thanks