This question can be answered from different users:
As a sysadmin I can update the service definition to update the classification, i can even create multiple classifications so a enduser can select the most relevant classification
As a enduser terriajs does not enable me to define a custom scale for a wms, it does allow me to select a relevant classification, if multiple classifications are available
Terriajs currently does not facilitate to load GeoTiff directly (although there are plans to support this case in the future), when GeoTiff is loaded directly, customisation of the styling will be facilitated. such functionality is for example available in https://felt.com/
how can the legend of a (grid) layer be adjusted on terria?