The Nexus 6 was one of the first devices to support Google's latest version of Android, Android 6.0 Marshmallow. The stock images have been out now for weeks. I have been running MM on my Nexus 6 for a few weeks now and so far it is pretty nice. If you are a custom rom user then you probably don't want to go through the hassle of flashing your phone back to stock, rooting it, and then reflashing your custom recovery.
If you would rather just flash a Rom based on Marshmallow you are in luck! Developer "zephiK" has just released his latest build of the Chroma rom for the Nexus 6 and it is based on MRA58K. The Chroma rom is focused on security, battery, performance, and stability. It is pretty much stock android with a few extra features. Chroma also provides you with the latest security patches to prevent vulnerabilities from being exploited on your device.
Rom features include a modified kernel, layers support built in, silent mode, Native LED notification, status bar customization, battery style, clock and date, brightness control, double tap status bar to turn off screen, show notification count, quick pull down left/right, network traffic, tile customizations, heads up notifications, Nav bar customizations, Lock screen customizations, Volume rocker settings, and more! Grab this rom from the link below.
via XDA