The heat map color ramp in ArcGIS Online is not customizable, unlike the ones offered in ArcMap and ArcGIS Pro. In ArcGIS Online Map Viewer, limited options of pre-configured heat map color ramps are available by clicking the Symbols icon in the Change Style setting, as shown below. There is no option to customize the RGB (red, green, blue) or Hex values in the Change Style setting.
This is a known limitation in ArcGIS Online. The RGB value of a heat map color ramp is not customizable in ArcGIS Online Map Viewer, unless configured through the ArcGIS REST Administrator Directory page.
The image below shows a pre-configured heat map color ramp by clicking the Symbols icon. Use the following workaround to customize the heat map color ramp in ArcGIS Online Map Viewer:
Note: To perform these steps, log in with administrator credentials.
Note: The alpha values range from 0 (fully transparent) to 255 (the default maximum value). Changing the maximum value to more than 255 is possible, but does not show any effect on the map. Ensure the alpha value of the first color stop is set to zero. Setting the alpha value of the first color stop to other than zero fills ArcGIS Online Map Viewer with a background color, as shown in the image below. Ensure the alpha value of a color stop (other than the first color stop) is set to other than zero. Setting alpha values of color stops to zero creates hollow rings, as shown in the image below.