intox18 You can actually add your own by combining gradient class with background class:
https://www.gotbootstrap.com/themes/smartadmin/4.4.5/plugin_smartpanels.html
When you initialize the panels, you can set or remove new colors.
panelColors: ['bg-primary-700 bg-success-gradient',
'bg-primary-500 bg-info-gradient',
'bg-primary-600 bg-primary-gradient',
'bg-info-600 bg-primray-gradient',
'bg-info-600 bg-info-gradient',
'bg-info-700 bg-success-gradient',
'bg-success-900 bg-info-gradient',
'bg-success-700 bg-primary-gradient',
'bg-success-600 bg-success-gradient',
'bg-danger-900 bg-info-gradient',
'bg-fusion-400 bg-fusion-gradient',
'bg-faded'],