Symbolism, Imagery, Allegory
Sometimes in books we're the ones placing symbolic meaning onto things, reading between the lines while characters go about their fictitious lives none the wiser that, say, every time they wear a black dress something terrible happens. This is decidedly not the case when it comes to feminine accessories and our girl Georgia, though—Georgia is so obsessed with growing up and being seen as adult that when it comes to the practices and accessories she associates with adulthood, she can't help but announce them as evidence of her own maturity.
Of course, the truth is that in her insistence that she is mature, Georgia shows us that she isn't. Consider her assessment of the significance of wearing a bra, for instance. She writes:
I am bursting with womanhood, I wear a bra! (1.1)
On one hand, Georgia is right: Physical changes are a very real part of growing up and coming of age. In this superficial sense, bras do indeed separate the girls from the women to a degree. But maturity is much more nuanced than physical changes—it's about understanding the world and your part in it, not about your cup size—and Georgia's inability to see beyond the physical markers of growing up lets us know that she actually has a lot of maturing left to do, no matter how her body has changed.
In the end, Georgia's decision to bleach a strip of her hair lets us know that she hasn't actually matured all that much over the course of this book. She dyes the same strip several times, trying to get it perfectly pale, but in the process totally fries her hair—to the point that the part she dyes falls out when she runs her fingers through it. Oops. If this strip of hair represents Georgia trying to present herself as a woman, then when it falls out we can see that, symbolically speaking, she has failed.
We're not knocking Georgia or anything. Remember: She's only fourteen… and there are nine more books in the series. In other words, girl's got plenty of time.