The links will appear grey when IRIS does not get any data for the links.
Links can be coloured by data (graphs) or status (alarms).
Another thing to remember is that the alarms need to be within the last 15min or the link will show grey.
Normally when a link is green to one person or group and not another, it would indicate that the person trying opening the map doesn't have access to view the data for that link. This would likely be a view setting on the link, not being a member or one of the children of the users own view.