Hello Morel

You would need to add it as custom CSS with Add Admin CSS plugin (for example). This is the code for pending:

.am-appointment-status-symbol.pending:before, .am-employee-status-symbol.pending:before {
background-color: #ffa700;
}

For Approved:

.am-appointment-status-symbol.approved:before, .am-appointment-status-symbol.paid:before, .am-employee-status-symbol.approved:before, .am-employee-status-symbol.paid:before {
background-color: #5fce19;
}

For Cancelled:

.am-appointment-status-symbol.canceled:before, .am-employee-status-symbol.canceled:before {
background-color: #ff1563;
}

And for Rejected:

.am-appointment-status-symbol.rejected:before, .am-employee-status-symbol.rejected:before {
background-color: #354052;
}

I hope that helps.

What do you mean, Morel, can you show me a screenshot of this button?

I don't understand why it's not working for you. That's the first line of code I provided:

.am-appointment-status-symbol.pending:before, .am-employee-status-symbol.pending:before {
background-color: #ffa700;
}

If it doesn't work, can you provide me a temporary wp-admin (administrator) user, so I can check this out? Remember this is a public ticket, so make sure to enable the PRIVATE response.