I’m adding two images, both of which are PNGs with transparency:
+ 
The result is:
sandcastle showing bug
This may be related to this issue: Cesium.ImageMaterialProperty issue with translucent PNG. The answer from @hannah was to use either transparent: true or color with an alpha value. However, I did not find an acceptable result.
Both translucent: true and color: new Color(1, 1, 1, 0.9999) produced a translucent effect:
Note: the blue is not translucent; it’s a solid color. And this is not 0.9999 opacity.
Is there a different way that I can get the desired effect? Note: I cannot use Viewer because I’m using ol-cesium so I only have a handle on Scene.