完整代码

import Foundation
import UIKit

extension UIColor {
    convenience init(r : CGFloat,g : CGFloat,b : CGFloat){
        self.init(red: r/255.0,green: g/255.0,blue: b/255.0,alpha: 1.0);
    }
    //简化RGB颜色写法
    class func RGBA(_ r : UInt,g : UInt,b : UInt,a : CGFloat) -> UIColor {
        let redFloat = CGFloat(r) / 255.0
        let green = CGFloat(g) / 255.0
        let blue = CGFloat(b) / 255.0
        return UIColor(red: redFloat,green: green,blue: blue,alpha: a)
    }
    //随机色
    class func randomColor() -> UIColor {
        return UIColor(r: CGFloat(arc4random_uniform(256)),g: CGFloat(arc4random_uniform(256)),b: CGFloat(arc4random_uniform(256)));
    }
    //16进制颜色
    class func colorWithHexString(_ hex: String,alpha : CGFloat = 1.0) -> UIColor {
        var hex = hex
        if hex.hasPrefix("#") {
            hex = hex.substring(from: hex.characters.index(hex.startIndex,offsetBy: 1))
        }
        
        // Deal with 3 character Hex strings
        if hex.characters.count == 3 {
            let redHex   = hex.substring(to: hex.characters.index(hex.startIndex,offsetBy: 1))
            let greenHex = hex.substring(with: (hex.characters.index(hex.startIndex,offsetBy: 1) ..< hex.characters.index(hex.startIndex,offsetBy: 2)))
            let blueHex  = hex.substring(from: hex.characters.index(hex.startIndex,offsetBy: 2))
            
            hex = redHex + redHex + greenHex + greenHex + blueHex + blueHex
        }
        
        let redHex = hex.substring(to: hex.characters.index(hex.startIndex,offsetBy: 2))
        let greenHex = hex.substring(with: (hex.characters.index(hex.startIndex,offsetBy: 2) ..< hex.characters.index(hex.startIndex,offsetBy: 4)))
        let blueHex = hex.substring(with: (hex.characters.index(hex.startIndex,offsetBy: 4) ..< hex.characters.index(hex.startIndex,offsetBy: 6)))
        
        var redInt:   CUnsignedInt = 0
        var greenInt: CUnsignedInt = 0
        var blueInt:  CUnsignedInt = 0
        
        Scanner(string: redHex).scanHexInt32(&redInt)
        Scanner(string: greenHex).scanHexInt32(&greenInt)
        Scanner(string: blueHex).scanHexInt32(&blueInt)

        return UIColor(red: CGFloat(redInt) / 255.0,green: CGFloat(greenInt) / 255.0,blue: CGFloat(blueInt) / 255.0,alpha: CGFloat(alpha))
    }
}

使用举例

RGBA

tableView.backgroundColor = UIColor.RGBA(53,g: 46,b: 54,a: 1)

self.tabBarController?.tabBar.barTintColor = UIColor.colorWithHexString("#252122")