Menu: View→Interface Properties→Colors→System Colors
Select a color for Highlighted objects, for Selected objects, or for the path diagram Background, depending on which of those three options is selected above.
Programming
In Amos Graphics programs, you can select the first item in this ComboBox as follows (Visual Basic):
Dim aComboBox As Windows.Forms.ComboBox = pd.GetComboBox("InterfacePropertiesForm", "ColorsCombo1")
aComboBox.SelectedIndex = 0