Thank you for writing.
Here is how to access the element responsible for the button border:
((BorderPrimitive)radSplitButton1.DropDownButtonElement.BorderElement).ForeColor = Color.Red;
and here is how to access the separator between the action and the arrow buttons:
radSplitButton1.DropDownButtonElement.ButtonSeparator.BorderColor = Color.Red;
I hope that you find this information useful. Should you have any other questions, do not hesitate to contact us.
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Feedback Portal
and vote to affect the priority of the items