Answer
Selecting a symbology category for a layer may require choosing a Color Scheme or Color Ramp.
The default ESRI style groups the default Color Ramps into four categories:
1. Default Ramps
2. Dichromatic Ramps
3. Spatial Ramps
4. Default Schemes
The Color Scheme dropdown on the Layer Properties > Symbology dialog lists all Color Ramp styles except for those in the Default Ramps and Dichromatic Ramps categories.
The Color Ramp dropdown on the Layer Properties > Symbology dialog lists all Color Ramp styles except for those in the Default Schemes and Dichromatic Ramps categories.