Hello,
What do the coloured items mean in the following screenshot?
I cannot remember how I did get this.
Thank you in advance for your help!
View Voice Colors
These are voice colors. View > Note and Rest Colors > Voice Colors
Each voice/stream has its own color.
EDIT: You beat me, Dan, but I leave my post as there’s a tiny bit of additional information. 
At the first glance, I thought they are the voice colours.
However, I repeated “Change voice to the previous voice on staff”.
Thus, they must be the first voice.
It is really strange.
How could I confirm the voice number?
Dorico counts voices per instrument, not per staff, so there are actually two voices on the marimba grand staff.
Better to change voice to a specific voice from the list of available voices in use, such as Upstem Voice 1.
Thanks!