Bug: Stroke color with 0pt width causes a switch to "outline" view.
When I have a path with a stroke of 0pt and I try to apply color, it will switch to "outline" view and won't allow me to go back to "GPU Preview", tell me something like "Can't render preview".
The only way to bypass the bug, is to save the file, close Illustrator, open it again and voila everything works.
Unfortunatelly, I can't reproduce the bug right now.
Apply a stroke color to a 0 pt stroke should increase the stroke to 1 pt.
Please accept my sincere apology for the inconvenience. Please get in touch with us at any of the other support channels – https://helpx.adobe.com/support.html . Since this is not a generic issue that we can reproduce at our end, we will need someone to look into your machine to figure out what is going on here.