need help with coloring

i colored my daughter's hair in march with loreal preference hair color les blondissimes, extra light ash blonde.  her hair was naturally a dark blonde. her hair now has a yellow tone to it and her roots have grown out almost two inches. what can i do to remove this color? she is wanting her hair darker(blonde) . i was told to use the same color and touch up her roots , just don'tleave it in that long. if i do that, won't i have to keep it up from now on?