It’s possible to do this, but it’s not a built-in feature. This is because the Color type in the User Defined Runtime Attributes panel creates a UIColor, but layer.borderColor holds a CGColorRef type. Unfortunately, there’s no way to assign a CGColorRef type in Interface Builder.
However, this is possible through a proxy property. See Peter DeWeese’s answer to a different question for a possible solution to this problem. His answer defines a category that allows a proxy color to be set through Interface Builder.