All natural hair colors will reveal the same underlying pigment when a decolorizing agent or lightener, aka bleach, is applied. Understanding this and training one's eye to identify these stages of change is invaluable for any colorist.
These are known as the stages of lightening:
If one lightens past the pale yellow stage, a white result, then the structural integrity of the hair has been comprimised. It is therefore recommended that if a pale, clear ash blonde result is desired, akin to Christina Aguilera, then lighten to pale yellow and follow with a violet-based toner to neutralize the unwanted yellow pigment. | 
