Is it possible to change the selection color in ArcGIS Online Map Viewer?

Answer

No. It is not possible to change the selection color from cyan to other colors in Map Viewer and Map Viewer Classic. It is by design that the selection color is not configurable.

Alternatively, create a web app using ArcGIS Web AppBuilder, and configure the Select widget in the web app. The selection color of the Select widget can be changed. Refer to ArcGIS Web AppBuilder: Configure the Select widget for the steps to configure the widget in a web app.

The option to change the selection color is also available in ArcGIS Pro as an application setting that applies to ArcGIS Pro projects. Refer to ArcGIS Pro: Selection options for more information.