multi colored screen when shutting down iMac

All,

Since upgrading to macOS Sierra, my iMac shows a multi colored screen when I shut down, see image below.

The screen shows up immediately after shut down starts, remains for a few seconds, than turns black and shuts down. It is not really a big deal, but it is bugging me. So, has anyone seen something similar? My iMac is a Retina 5K, 27-inch, Late 2015 model running macOS Sierra 10.12.

Wil Dieteren