This is nowhere explained and hence solely assumed. See explanation in Set a Handle's Halo. A converter to set a value to this format has only been added in Titan v11.2.

This is supposed to work:

ActionScript.SetProperty(“Handles.PendingHalo”, colour:“#ffff0000”)

The prefix for this cast will be “colour” and value can either be a hexadecimal string or an integer value (not hexadecimal).