Every account has its own color.
Several account share the same colour. 3 in my case, but the the 3rd is out of side at the screenshot

0.12.0 Beta2
Mailserver or service:
Freenet, GMX
Operating system: Ubuntu 18.04 HWE
Web server: Nginx
Database: MariaDB
PHP version: 7.2
Nextcloud Version: 15.0.4
Browser: FF65, Chromium 71.0
Operating system: Ubuntu 18.10
I've seen that as well. This seems to be a bug as it used to work when I rewrote that code.
If anybody is interested in helping with a fix you can find the code here: https://github.com/nextcloud/mail/blob/202c442a163966122378eb75f0f94450dbb4cdb8/src/mixins/SidebarItems.js#L21and https://github.com/nextcloud/mail/blob/202c442a163966122378eb75f0f94450dbb4cdb8/src/util/AccountColor.js#L25-L31
Oh, I think I know what the bug is. We use the acount name (e.g. "Christoph Wurst") and not the email address for the color. That's why you see two accounts with the same color as the name is identical ;)
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Oh, I think I know what the bug is. We use the acount name (e.g. "Christoph Wurst") and not the email address for the color. That's why you see two accounts with the same color as the name is identical ;)