dying medium brown hair with gray to red

trying to help my wife go redder. she has medium brown hair now (colored) but is going gray so the roots are fairly white. last time she did color she applied the color to the roots first then lengths and came out with hot roots. they have since faded but i'd like to avoid hot roots next time she colors.  any suggestions? this is color from madison reed - trieste red.

nobody can give me some advice- or direct me to some professional hair coloring forums??