Thanks to Jim Herberich who provided the following code, which I adapted,
and to Jim Scace who provided code for an alternative solution.

David:

I ran into the same problem you have, but I was dealing with Raster
Catalogs.  The fix should be the same:

Sub TransColorRenderer()

Dim pMxDoc As IMxDocument
Set pMxDoc = ThisDocument

Dim pLy As ILayer
Set pLy = pMxDoc.SelectedLayer

Dim pCatLy As IRasterLayer
Set pCatLy = pLy

Dim pCatRend As IRasterUniqueValueRenderer
Set pCatRend = pCatLy.Renderer

Dim pSym As IColorSymbol
Set pSym = New ColorSymbol

Dim pColor As IRgbColor
Set pColor = New RgbColor

pColor.NullColor = True
pSym.Color = pColor

Dim pNumbDlg As INumberDialog
Set pNumbDlg = New NumberDialog

Dim boolValid As Boolean
boolValid = pNumbDlg.DoModal("Colormap Entry:", 255, 0, Application.hWnd)

If boolValid Then
    pCatRend.Symbol(0, CInt(pNumbDlg.Value)) = pSym
    pMxDoc.ActiveView.Refresh
End If

End Sub

------------------------------------------------------
Jim Herberich               mailto:jherberich@ensr.com
ENSR International
2 Technology Park Drive           voice (978) 589-3193
Westford, MA 01886                  fax (978) 589-3100

Original message:

AO users:

I have a set of 1:24000 USGS topo maps for Washington State (tiff format)
that have had the collars clipped so that they can be used in a seamless
mosaic.  I have worked out the code to add these tiles to ArcMap by
hotlinking them to an index layer.  But the tiles are displayed with a solid
color in the border area where the collars have been clipped.  It's simple
to manually change the symbol for this border (value = 255) to "no color" in
ArcMap but I would like to do it programmatically as each tile is added.  I
have tried everything I can think of with rasters, raster layers, raster
bands, raster renderers, etc. but am stymied at this point.

Can anyone point me in the right direction?

Thanks!


********************
David F. Fox, PE
GIS Coordinator

Information Management Office
U.S. Army Corps of Engineers
Seattle District

Phone:  206-764-6083
Fax:  206-764-6816
Email:  david.f.fox@usace.army.mil

**********************************