You can refer to a woman whose hair has been artificially been made lighter in colour as a peroxide blonde, especially when you want to show that you disapprove of this, or that you think her hair looks unnatural or unattractive.
peroxide blonde in British English
noun
usually derogatory
a woman having hair that is bleached rather than naturally blonde and that looks harsh or unnatural
Examples of 'peroxide blonde' in a sentence
peroxide blonde
Going peroxide blonde is the quickest way to give yourself a completely fresh start.