We don't talk about down votes on hn "because it is boring" to dang. And because there is an incentive to downvote something which is downvoted. Lighter color indicates downvoted content. It shows reputation. As reputation precedes, people will downvote lighter colored text until someone asks why below it. In which case they will reconsider.
There are also a lot of HNers (myself included) who will upvote any comments they see that they think are unfairly downvoted, whether they agree or disagree with the comment. It creates a pretty good balancing effect. The end result is that comments that stay downvoted after having a decent number of eyeballs on them seem to usually deserve it.
A possible exception are hyper-contentious issues (Israel/Palestine for example), where the upvote/downvote count is just a reflection of which tribe has greater numbers on HN.
This is a misunderstanding in how downvotes work here.