UIColor+HexString.m

// UIColor+HexString.h

#import <UIKit/UIColor.h>

NS_ASSUME_NONNULL_BEGIN

@interface UIColor (HexString)

+ (UIColor *) colorWithHexString: (NSString *) hexString;
+ (CGFloat) colorComponentFrom: (NSString *) string start: (NSUInteger) start length: (NSUInteger) length;

@end

NS_ASSUME_NONNULL_END

// UIColor+HexString.m

#import "UIColor+HexString.h"

@implementation UIColor (HexString)

+ (UIColor *) colorWithHexString: (NSString *) hexColor {
  NSString *hex = [hexColor stringByReplacingOccurrencesOfString: @"#" withString: @""];
  NSString *colorString = [hex uppercaseString];
  CGFloat alpha;
  CGFloat red;
  CGFloat blue;
  CGFloat green;

  switch ([colorString length]) {
    case 3:
      // RGB
      alpha = 1.0f;
      red   = [self colorComponentFrom: colorString start: 0 length: 1];
      green = [self colorComponentFrom: colorString start: 1 length: 1];
      blue  = [self colorComponentFrom: colorString start: 2 length: 1];
      break;
    case 4:
      // #ARGB
      alpha = [self colorComponentFrom: colorString start: 0 length: 1];
      red   = [self colorComponentFrom: colorString start: 1 length: 1];
      green = [self colorComponentFrom: colorString start: 2 length: 1];
      blue  = [self colorComponentFrom: colorString start: 3 length: 1];
      break;
    case 6:
      // #RRGGBB
      alpha = 1.0f;
      red   = [self colorComponentFrom: colorString start: 0 length: 2];
      green = [self colorComponentFrom: colorString start: 2 length: 2];
      blue  = [self colorComponentFrom: colorString start: 4 length: 2];
      break;
    case 8:
      // #AARRGGBB
      alpha = [self colorComponentFrom: colorString start: 0 length: 2];
      red   = [self colorComponentFrom: colorString start: 2 length: 2];
      green = [self colorComponentFrom: colorString start: 4 length: 2];
      blue  = [self colorComponentFrom: colorString start: 6 length: 2];
      break;
    default:
      [NSException raise:@"Invalid color value" format: @"Color value %@ is invalid.  It should be a hex value of the form #RBG, #ARGB, #RRGGBB, or #AARRGGBB", hexColor];
      break;
  }

  return [UIColor colorWithRed: red green: green blue: blue alpha: alpha];
}

+ (CGFloat) colorComponentFrom: (NSString *) string start: (NSUInteger) start length: (NSUInteger) length {
  NSString *substring = [string substringWithRange: NSMakeRange(start, length)];
  NSString *fullHex = length == 2 ? substring : [NSString stringWithFormat: @"%@%@", substring, substring];
  unsigned hexComponent;
  [[NSScanner scannerWithString: fullHex] scanHexInt: &hexComponent];
  return hexComponent / 255.0;
}

@end