【Swift+Extension】UILabelで、一部の文字だけ色を付ける ref: https://qiita.com/Arime/items/5ede5f9423a205cf2deb

file2.swift

let aLabel = UILabel()
aLabel.text = "あいうえお"

// "あいう"を赤色にする場合
aLabel.addAccent(pattern: "あいう", color: .red)

String+Range.swift

import Foundation

extension String {

    func nsRange(from range: Range<String.Index>) -> NSRange {
        return NSRange(range, in: self)
    }

    func ranges(of searchString: String, options mask: NSString.CompareOptions = [], locale: Locale? = nil) -> [Range<String.Index>] {
        var ranges: [Range<String.Index>] = []
        while let range = range(of: searchString, options: mask, range: (ranges.last?.upperBound ?? startIndex)..<endIndex, locale: locale) {
            ranges.append(range)
        }
        return ranges
    }

    func nsRanges(of searchString: String, options mask: NSString.CompareOptions = [], locale: Locale? = nil) -> [NSRange] {
        let ranges = self.ranges(of: searchString, options: mask, locale: locale)
        return ranges.map { nsRange(from: $0) }
    }

}

UILabel+Accent.swift

import UIKit

extension UILabel {

    /// 対象の文字列に対して、アクセント色を付加する
    ///
    /// - Parameters:
    ///   - pattern: 対象の文字列
    ///   - color: アクセント色
    func addAccent(pattern: String, color: UIColor) {
        // String
        let strings = [attributedText?.string, text].flatMap { $0 }
        guard let string = strings.first else { return }

        // Ranges
        let nsRanges = string.nsRanges(of: pattern, options: [.literal])
        if nsRanges.count == 0 { return }

        // Add Color
        let attributedString = attributedText != nil
            ? NSMutableAttributedString(attributedString: attributedText!)
            : NSMutableAttributedString(string: string)

        for nsRange in nsRanges {
            attributedString.addAttributes([.foregroundColor: color], range: nsRange)
        }

        // Set
        attributedText = attributedString
    }

}