On US a yellow screen comes up if you have an exit row... it's just to remind the GA to verify that the pax meets the FAA regulations for sitting there (18 or older, not traveling with a child or somebody needing assistance, no obvious physical impairments, etc...)

The red (or "reject") screen means your seating assignment changed from what's on your BP... frequently happens when an elite is downgraded from F to accommodate a FAM or paying F pax.