Currently it's default is CMYK, no matter how many times RGB is selected next time it's opened CMYK default again.
If the color mode of the document is CMYK, the default value is CMYK. Make sure that you choose RGB as default color mode, under Tols/color management
"Primary Color Mode" thanks!
I think this depends on the styles that are being used for default object properties. You can find those in the Object Styles docker.
If you start a new document by using the "Create a New Document" dialog, and specify RGB as the primary color mode in that dialog, then styles for default object properties in that document come from a file named "rgb.cdt" that is contained in the active workspace file.
If you then change the primary color mode of the document to CMYK, it does not change the styles for default object properties in the document. They are still styles that came from rgb.cdt.
By using "Save Settings as Default", one can change the default object properties that are stored in the rgb.cdt or cmyk.cdt files, and are used when creating new documents. It is possible to have default object properties for new documents that are totally inconsistent with the primary color mode of the document. If this happens to a user by accident, the results can be very confusing.
Instead of using the "Create a New Document" dialog, I usually start new documents based on templates. For me, it is easier that way to know exactly what the properties - including the styles for default object properties - are in a document I am creating.