RE: Colours and 3D parts HoloLens 2
Hi Martina, currently you cannot delete imported items directly from the PC app but you can delete or deactivate them from the Dynamics 365 Guides Portal on your Dynamics instance. If you don't know the URL of your instance, do this: Select the Analyze tab in the PC app. The URL is shown in the Instance URL field at the bottom of the page (highlighted in red below). Copy this URL and paste it into the address bar of your web browser.
Log into your instance with your admin credentials. You should see a box for Guides under Published Apps:
Browse into that and look for 3D on the left-hand menu:
You will then see a table of objects. Browse down and select the object you want to remove. The toolbar should have options to Deactivate and Delete. If the toolbar doesn't refresh, reselect the item or double-click into the item. Use Deactivate if you want to restore later, use Delete if you want to remove it permanently.
For your other issue (HoloLens 2 colors), have you tried adjusting the brightness using the buttons on the left side of the device? It may help to reset the device and try again.