You can easily mark and group your customers by using the standard field Company Category or Contact Types for contacts.
Go to Settings - CRM - Companies - Company Category or Settings - CRM - Contact - Contact Category. You can specify the type of company / contact you are going to work with, for example: end customer, VIP customer, retailer, wholesaler, haulage contractor, limited liability company or trading company....
These categories / types of your choice will then be available in the drop-down menu on the company card or contact card when you edit your companies or contacts.
You can specify a colour for each company category or contact type by clicking on the colourful square on the row. This will make it easier to identify different categories at a glance in the list.
Click on the bin icon at the end of the row to remove a categogy or type. Any categories or types that are in use cannot be deleted.