NSAttributedString (and its mutable sibling NSMutableAttributedString) allows you to create strings that are complex in their appearance to the user.

A common application is to use this to display a string and adding custom kerning / letter-spacing.

This would be achieved as follows (where label is a UILabel), giving a different kerning for the word "kerning"

Swift

var attributedString = NSMutableAttributedString("Apply kerning")
attributedString.addAttribute(attribute: NSKernAttributeName, value: 5, range: NSMakeRange(6, 7))
label.attributedText = attributedString

Objective-C

NSMutableAttributedString *attributedString;
attributedString = [[NSMutableAttributedString alloc] initWithString:@"Apply kerning"];
[attributedString addAttribute:NSKernAttributeName value:@5 range:NSMakeRange(6, 7)];
[label setAttributedText:attributedString];

Objective-C

NSMutableAttributedString *attributeString = [[NSMutableAttributedString alloc] initWithString:@"Your String here"];
[attributeString addAttribute:NSStrikethroughStyleAttributeName
                    value:@2
                    range:NSMakeRange(0, [attributeString length])];

Swift

let attributeString: NSMutableAttributedString =  NSMutableAttributedString(string: "Your String here")
attributeString.addAttribute(NSStrikethroughStyleAttributeName, value: 2, range: NSMakeRange(0, attributeString.length))

Then you can add this to your UILabel:

yourLabel.attributedText = attributeString;

let someValue : String = "Something the user entered"     
let text = NSMutableAttributedString(string: "The value is: ")
text.appendAttributedString(NSAttributedString(string: someValue, attributes: [NSFontAttributeName:UIFont.boldSystemFontOfSize(UIFont.systemFontSize())]))

The result looks like:

The value is: Something the user entered

Objective-C

UIColor *color = [UIColor redColor];
NSString *textToFind = @"redword";

NSMutableAttributedString *attrsString =  [[NSMutableAttributedString alloc] initWithAttributedString:yourLabel.attributedText];

// search for word occurrence
NSRange range = [yourLabel.text rangeOfString:textToFind];
if (range.location != NSNotFound) {
    [attrsString addAttribute:NSForegroundColorAttributeName value:color range:range];
}

// set attributed text
yourLabel.attributedText = attrsString;

Swift

let color = UIColor.red;
let textToFind = "redword"
        
let attrsString =  NSMutableAttributedString(string:yourlabel.text!);
        
// search for word occurrence
let range = (yourlabel.text! as NSString).range(of: textToFind)
if (range.length > 0) {
     attrsString.addAttribute(NSForegroundColorAttributeName,value:color,range:range)
}
        
// set attributed text
yourlabel.attributedText = attrsString

Note:

The main here is to use a NSMutableAttributedString and the selector addAttribute:value:range with the attribute NSForegroundColorAttributeName to change a color of a string range:

NSMutableAttributedString *attrsString =  [[NSMutableAttributedString alloc] initWithAttributedString:label.attributedText];
[attrsString addAttribute:NSForegroundColorAttributeName value:color range:range];

You could use another way to get the range, for example: NSRegularExpression.

Objective-C

NSMutableAttributedString *mutAttString = @"string goes here";
NSRange range = NSMakeRange(0, mutAttString.length);
[mutAttString setAttributes:@{} range:originalRange];

As per Apple Documentation we use, setAttributes and not addAttribute.

Swift

mutAttString.setAttributes([:], range: NSRange(0..<string.length))