What is the UIColor of the default UITableView separator?

The color is not guaranteed to be a specific color. It can be changed over OS and SDK versions.
You can retrieve exact color dynamically by accessing separatorColor property.

UITableView* TV = [[UITableView alloc] init];
UIColor* C = [TV separatorColor];
CGColorRef CGC = [C CGColor];

Now you can get the each channel values through UIColor‘s methods. Or use the CGColor directly for drawing.

Here’s header file comment of the property in UITableView.h.

@property(nonatomic,retain) UIColor *separatorColor;
// default is the standard separator gray

If you want to avoid instantiation cost of UITableView for each time, just get it once and cache it.

As @Isuru noted in comment, you can write in Swift like this.

UITableView().separ‌​atorColor

As @Jordan noted in comment, you also can store the result to avoid further evaluation cost.

let defaultTableSeparato‌​rColor = UITableView().separa‌​torColor