A style added to a drop-down list field does not show in the form. However, the style appears in the column list and when you hover over the reference icon in the list.
Steps to Reproduce
Log in to a demo Fuji or Geneva instance.
Go to Projects > All.
Note that the Risk column shows colors next to the entries. If you hover the mouse cursor over the reference icon for a project, the Risk field shows in color.
Click the project.
Note that the Risk field in the project form does not show in colour.
This is expected behaviour and by design. See also the related article Value-based field styles applied to forms inconsistently on UI14 and UI15.
Related Problem: PRB672075