I assume we must be talking about legacy form fields, in which case you will need a macro run on exit from the checkbox field and you would have to leave the field to run it e.g.
Code:
Sub MacroOnExit()
If GetCurrentFF.CheckBox.Value = True Then
GetCurrentFF.Range.Font.ColorIndex = wdAuto
Else
GetCurrentFF.Range.Font.ColorIndex = wdRed
End If
lbl_Exit:
Exit Sub
End Sub
Private Function GetCurrentFF() As Word.FormField
Set rngFF = Selection.Range.FormFields(1).Range
Set GetCurrentFF = rngFF.FormFields(1)
lbl_Exit:
Exit Function
End Function