UIView's border color in Interface builder doesn't work?

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.

You have to create Category for CALayer:

CALayer+UIColor.h

#import <QuartzCore/QuartzCore.h>
#import <UIKit/UIKit.h>

@interface CALayer(UIColor)

// This assigns a CGColor to borderColor.
@property(nonatomic, assign) UIColor* borderUIColor;

@end

CALayer+UIColor.m

#import "CALayer+UIColor.h"

@implementation CALayer(UIColor)

- (void)setBorderUIColor:(UIColor*)color {
    self.borderColor = color.CGColor;
}

- (UIColor*)borderUIColor {
    return [UIColor colorWithCGColor:self.borderColor];
}

@end

And then in User Defined Runtime attributes You can use it as it is on image below:

For Swift it is much more simple:

@IBInspectable var borderColor: UIColor? {
    didSet {
        layer.borderColor = borderColor?.CGColor
        layer.borderWidth = 1
    }
}

Then in Xcode you can use it like this:

Once you choose sth it is automatically added to your runtime attributes:

Copy and paste this class:

import UIKit

@IBDesignable class BorderView : UIView {
    @IBInspectable var borderColor: UIColor = .clear {
        didSet {
        layer.borderColor = borderColor.cgColor
        }
    }

    @IBInspectable var borderWidth: CGFloat = 0 {
        didSet {
            layer.borderWidth = borderWidth
        }
    }

    @IBInspectable var cornerRadius: CGFloat = 0 {
        didSet {
            layer.cornerRadius = cornerRadius
        }
    }
}

Now in Interface Builder, go to the Identity inspector and set your view as a CustomView class.

After that, check out your Attributes Inspector:

No need to mess around with user defined runtime attributes anymore. And your changes will also show up on the canvas!