I'm not sure what the desired behavior is in this situation, so I'm opening this to know either way.
When the user copies formatted text and then pastes it into VisualEditor, VE does its best to preserve the formatting and converts it to the corresponding wikitext.
However, some types of formatting (e.g. text color) are discouraged on most Wikimedia sites. In [[ https://it.wikipedia.org/wiki/Speciale:Diff/71700137 | this example edit ]], the user inserted (and saved with VE) text that was underlined and colored blue. Underlining is usually discouraged in articles, but the user could have removed it using the VE toolbar. However, the toolbar doesn't allow them to change the text color, so they wouldn't have been able to fix it even if they had noticed it.
One possible solution could be to keep formatting that is supported by the toolbar, and to strip what can't be edited.