I see.. Interestingly, the default rule in my Resharper does the opposite (always recommend underscore for fields and colour the code if it doesn't have one). In fact my habit switched from no underscore to underscore because of Resharper and I think it's also more useful in avoiding name clash between field and method parameter :)

If the code consistently has no-underscore behaviour, I'll change it and change my resharper rule for this project.