Dispatching calls in Ada and C++ source code are highlighted by default
in GPS via the dispatching.py plug-in.
Based on the cross-reference information, this plug-in will highlight (with a special color that you can configure in the preferences dialog) all calls that are dispatching (or calls to virtual methods in C++). A dispatching call, in Ada, is a subprogram call where the actual subprogram that is called is not known until run time, and is chosen based on the tag of the object (so this of course only exists when you are using object-oriented programming).
To disable this highlighting (which might sometimes be slow if you are
using big sources, even though the highlighting itself is done in the
background), you can go to the /Tools/Plug-ins menu, and
disable the dispatching.py plug-in.