The original Moto X was launched with Moto Maker customization options, and the second iteration expanded the selection of materials. Through it all the modest Moto G has been mostly left out of the fun, but a leak from Evan "@evleaks" Blass points to possible Moto Maker support for the upcoming Moto G refresh.
The image posted by Blass shows the phone in a variety of colors, which by itself isn't unusual. The Moto G has swappable back covers already, but this image shows different colors for the metal accents as well. That's something you can customize on the Moto X in Moto Maker. It's unclear if those will still be removable going forward.
If the schedule of past years holds, we should get a new Moto G announcement in late August or early September.