In HOOPS Communicator you can change the selection color using Communicator.SelectionManager.setNodeSelectionColor and setNodeSelectionOutlineColor. However, these API’s are limited to a single color for selection. If you want to use multiple colors to highlight objects, you can achieve this by bypassing the selectionManager and setting colors directly using Communicator.Model.setNodesLineColor and setNodesColors. When there is no selection, reset the color using resetNodesColor.
Be aware that setting colors directly will be slower and will generally cause a scene refresh vs utilizing the SelectionManager for highlighting.
Please see the HOOPS Communicator Reference Manual for more info:
Model — HOOPS Communicator documentation