Currently there is only support for a standard set of colour ramps in ArcGIS Dashboards:

The base set does not allow for custom colour ramps added by the users.
It would be great to have the ability to create my own colour ramp. I have over 100 unique records, so I'd like to apply it to specific data values.
Similar idea to ArcGIS BowerBI post: Divergent or custom colour ramps... - Esri Community
@NathanEnge