ARP list is static, can not see the client in not online, that can make the color distinguish.
ARP entry is removed from the list, once timeout is reached.
If it is important to check the status of user, create network monitor (by The Dude) and monitor all the hosts there.
Too much trouble, it is better to bring identification
In my opinion ARP table is not correct place, where you should monitor whether user is online or not,as ARP timeout is low.
ARP table timeout is 15-30 seconds, and even if 10 seconds are used as identification of user online status. Then your ARP table with >100 hosts will be like Christmas tree, that blinks all the time.
Please use Network monitor to check host status (online/offline)