Usually a grey bar means the site is not indexed in google or has been banned so links from these have little or no value (as google isn't picking them up) PR 0 sites, these will give some value as long as the link is relevant. All links on pr 0+ are worth having in case one day the PR of the page increases, then you'll get more value.
grey bar doesnt mean you've been banned or not indexed. Page rank in the bar is updated more than it used to be, but it still isn't always updated! grey bar and pr0 sites will give you some good links providing they are relevant, unless this site is clearly advertising that you are paying for them. Don't ever chase links just for pr unless you are thinking about selling them eventually A pr0 link on a related site will be much better than a pr5 link on an unrelated site!