Run-Time error when calling GanttChartFormat from Application Event (Project Professional 2010)

Newbie here so please forgive any formatting oversights.

I'm attempting to use an application event to have a change to a task's field automatically update the formatting on the Gantt chart bar.

Debugging shows that the event is handled correctly and passes the proper arguments, but I get Run-timer error '1100': "The method is not available in this situation." when the GanttBarFormatEx method is called:

' In a separate Application Events class that initializes and passes values correctly:
Private Sub App_ProjectBeforeTaskChange2(ByVal tsk As Task, _
                                         ByVal Field As PjField, _
                                         ByVal NewVal As Variant, _
                                         ByVal Info As EventInfo)

    Debug.Print "App_ProjectBeforeTaskChange2"
    If Field = pjTaskText4 And NewVal <> "" Then
        UpdateColors tsk, NewVal
    End If

End Sub

' ---------------------------------------------------------
' Troublesome code:

Sub UpdateColors(tsk As Task, sText As Variant)

    Dim pColor As PjColor

        Select Case sText
            Case "Red": pColor = pjRed
            Case Else: pColor = pjBlue
        End Select
	debug.print tsk.ID, pColor
        tsk.Application.GanttBarFormat TaskID:=tsk.ID, endcolor:=pColor

End Sub

I've tried the GanttBarFormat method as well (with and without the preceding tsk.Application) and both App_ProjectBeforeTaskChange & App_ProjectBeforeTaskChange2 but to no avail.

Also, calling the format method in a "dummy" sub works fine when using the same arguments that are passed to it by the app event.

A cursory search has turned up a few hits on MSDN and other sites, but I haven't found anything that helps.

It seems I'm not allowed to insert links yet so here is a relevant MSDN post:

https://social.msdn.microsoft.com/Forums/en-US/7a606e57-9990-4f59-8911-44a68d3e878a/the-method-is-not-available-in-this-situation-vba?forum=project2010custprog

Thanks in advance for any help!

-TD