By putting overlapping viewsheds and view domes into the same analysis layer, the coloring of the geometry is cumulated:

You can specify the color scheme in the Inspector of the Analysis Layer as follows:
- Visible by multiple—This color is applied to geometry that is visible from more than one observer point.
- Visible by one—This color is applied if the geometry is visible from one observer point only.
- Not visible by any—This color is applied if the geometry is not visible by any observer point. This color is also used to highlight geometry that violates a view corridor.
Tip:
It is recommended that you put view corridor objects in a separate analysis layer, as they cannot interact with the viewshed or view dome objects.