Hi GrinGEO,
Since you're saying the exported elevation GeoTIFF DEM is shown to you as a grayscale TIFF in a GIS, I'm assuming based on this that you use QGIS?
If so, that grayscale visualisation is the fault of your settings in QGIS. Not of the file created from Photoscan.
When viewed in a Global Mapper, it automatically colors the DEM based on the elevation values. QGIS doesn't do this by default when you load the file into it.
What you need to generate, for your client, is a simple PDF. As is it's a vector file the DEM will look as sharp as it looks in Photoscan.
Both QGIS and Global Mapper can export an elevation GeoTIFF as a PDF file. Perhaps even Photoscan can export it, not sure? (not talking about a 3D Model PDF!!) However in QGIS you first need to properly setup the visualisation before it looks as it does in Photoscan (Or Global Mapper).