Symbolism, Imagery, Allegory
Nope, we're not talking about Alice Walker's Pulitzer Prize winning book. Madam Walker aside, though, there's no denying that purple is a big deal in this book. It first comes to our attention when Gratuity tells us about her mom's mole:
The mole was only purple now, no longer flashing. Just bright and steady, and I will hate the color purple for the rest of my life. (2.158)
If you've read up on the mole elsewhere in this section, then you know it's implanted in Tip's mom by the Boov in order to listen in on the world and control her. So since the mole is purple, we should probably start thinking that purple's a suspicious color.
It's not just in the context of the mole that the color purple is bad news, though. As we learn over the rest of the novel, purple is the color of the Boov spaceship that sucks up Tip's mom and the hue of the Gorgs' spaceship, too. Purple, it seems, is the preferred color for invasion ships throughout the intergalactic community. The color purple, then, reminds Gratuity of the alien invasion, and being separated from her mom.