The "Save page" in the toolbar (i.e. the one that opens the save dialog, not the one that finalizes the save) should be 'progressive' (see style guide). In current skins (e.g. Vector) 'progressive' is rendered as a blue background.
Basically, 'progressive' (blue) means you're continuing the process (e.g. opening a dialog), while 'constructive' (rendered as green in current skins) means you're finalizing the process and taking a non-negative action (e.g. saving a page).
We want to maintain this distinction in general, but it's particularly important here because it uses the same text for both.
I understand that VisualEditor is not actually using the mediawiki.ui modules, but my understanding is the general idea is to be consistent with the style guide.