Sometimes it’s frustrating not to be able to alter the display of basemaps provided by ArcGIS Online. In this case, I was using “World Shaded Relief” (on account of its high, SRTM-derived resolution, but didn’t want its reddish hue. Further, although the transparency of an AGOL layer can be altered, other display properties (brightness, contrast, resampling) cannot be changed.
So this article came to the rescue: Creating a grayscale version of any ArcGIS Online basemap. It’s somewhat convoluted, involving the creation of a Mosaic Dataset, so I won’t try to repeat the method here. However, it did work, and I was able to alter the colour balance and contrast.
I found a further benefit: after adding the new layer and changing the display as needed, it was then possible to export the data to create a local copy, available for off-line / low bandwidth use.