You may notice that sometimes, the linked items have Gray Background on the Traceability Tab. GRAY here means that there is a linked item which is not available in the current view (project and version) where user login to.

Let’s review the different cases

,
Case 1:
Suppose you are working with different versions of the Project

Project A has 3 versions 1.0, 2.0 and 3.0

You have links from an item in (2.0) to Item in (1.0) and Item in (3.0)

You login to (2.0)

So, the row having a link to the item in (3.0) would appear grey

 Case 2:

Suppose you trace the Item of Project A to item in Project B (while you login to a solution)

Now you login to project A

Since project B is not in the view, the link to the item in project B will have Grey background