One way is using Color Set.

Start with creating new set in your xcassets catalog

... and name it how you want to

Then change your color to color that you need

Finally, set color of your label as this color from xcassets catalog

... or programmatically

label.textColor = UIColor(named: "ColorForLabel")

Now when you need to change color of text, just change color of this Color Set

try this in your AppDelegate.swift inside didFinishLaunchingWithOptions function:

UILabel.appearance(whenContainedInInstancesOf: [UIView.self]).textColor = .red //or what color you want

I really liked Arash Etemad's solution, but found it quite extreme as it overrode ALL colours, even if some of them were customised. That's not great when working on an existing large project.

So I came up with this (Swift 5.2):

extension UILabel {

    override open func willMove(toSuperview newSuperview: UIView?) {
        super.willMove(toSuperview: newSuperview)

        guard newSuperview != nil else {
            return
        }

        if #available(iOS 13.0, *) {
            if textColor == UIColor.label {
                textColor = .red
            }
        } else if textColor == UIColor.darkText {
            textColor = .red
        }
    }    
}

It uses the labels own lifecycle event to override default system font colour. With the appearance of dark mode in iOS 13, that can be identified as UIColor.label, whereas before it's UIColor.darkText.

This prevents a custom font colour from being overriden (unless your custom colour is the same as the default ?!? ), whilst not requiring to manually set font colour all over the project.