It will be common to have many attributes in the same feature class that may be relevant to a single asset and not across the class - such is the nature of the beast here. For usability, when setting up a display or app, I will typically hide that attribute from all assets that don't benefit from it. It's time consuming for initial setup, but does certainly improve the end user experience.
Assuming you have the Hydrant_Color attribute added to this feature class, using it to drive symbology is pretty straightforward. Add your Subtype Group Layer to the map, right click on the appropriate Asset Group in the table of contents and select symbology from the context menu. From here, you'd setup your symbology just the same as you would any other layer in Pro (SGL or not).
