Pick a color for any inbox you create and it follows that inbox everywhere: the folder in your sidebar and the tag on every conversation, so you can tell Sales from Service at a glance.
There are nine colors: Indigo, Blue, Teal, Purple, Charcoal, Green, Gold, Red, and Plum. Leave it on Default to keep the classic look.
The sidebar folder fills in that color, so a colored inbox is easy to spot even when it is not the one selected.
Conversation tags match. A thread in a colored inbox carries the same color in every list, and changing the color updates them right away.
How to set a color
In the Inbox, select the inbox you want to color under My Inboxes.
Click the ⋯ button that appears next to its name.
Choose a swatch under Color and save.
New inboxes get the same option: click + next to My Inboxes to create one and pick a color as you go.