To have a clearer view of your clients (e.g. separating them into prospects, regular clients, etc. or another classification of your choice), you can classify them using coloured labels.
First of all, you have to define your labels under Settings > Labels > Clients.
Then, to assign a label to a client, you have to click on '+add labels' in the client concerned, next to the name of the client. Just select the label(s) you would like to apply to this client, and click on Apply.
The label will appear next to the name of the client in the clients list.
You can also filter your list of clients or your reports based on these labels.