I see you have answered the question somewhat, but to provide a slightly more concise way without using regex to answer to the title question:

To change the colour of a length of text you need to know the start and end index of the coloured-to-be characters in the string e.g.

var main_string = "Hello World"
var string_to_color = "World"

var range = (main_string as NSString).rangeOfString(string_to_color)

Then you convert to attributed string and use 'add attribute' with NSForegroundColorAttributeName:

var attributedString = NSMutableAttributedString(string:main_string)
attributedString.addAttribute(NSForegroundColorAttributeName, value: UIColor.redColor() , range: range)

A list of further standard attributes you can set can be found in Apple's documentation