Click Manage
Model
(Model Explorer) in the Toolbar to open the Model Explorer panel.
Click
for the feature class or subset to highlight.

By default, the Activate Highlighting icon turns a light purple color when activated, and members of that feature class or subset are highlighted that color in the model.
On the Model Explorer, right-click any feature type to open the context menu.
Click Set Highlighting Color.
Select a color and click OK.
The Activate Highlighting icon color changes to the color you specified: a dark version when highlighting is off, and a bright version when highlighting is active.