The Dashboard->Transfers tab utilizes color to assist you in assessing the status of your backup accounts. Depending on whether the "uploads" or "downloads" boxes are ticked (as documented here and shown in screen capture below) the Transfer's tab will show the latest upload/download for each client and color code each line in the table (each computer's status) as described below.
A white row on the Transfers tab indicates that the client is currently connected and performing the upload/download operation.
A light-green row indicates that the client completed the operation successfully (i.e there were no warnings or errors).
A light-yellow row in the table indicates that the client was performing an operation but last sent a status update to the server between 20 and 60 minutes ago.
A yellow row indicates that the client completed the operation but there were errors or warnings which you can look up on the Dashboard->History tab.
A red line indicates that either the client did not finish its backup (it was interrupted, such as by clicking the STOP button or shutting down the client's computer), it completed with a fatal error, or it was performaning an upload/download operation but hasn't connected to the server in over an hour so its likely been interrupted. The Stage column will tell you the last status information the client sent to the server. Visiting the the Dashboard->History tab for the client may provide more information, but if the client was truely interrupted then you will have to wait for their next backup for log and status tables to be synchronized to server, or remote into the client to look at logs there.
The screen capture above shows how you can specify that client rows in the table be colored light-blue if they have not backed up in the number of specified days. The server has the capability to alert clients when do data or no connections are made in a specified (per client) number of days, which is documented here and here.