random color from an array for a different string

I'd like to pick random number for a different string. The following example is similar to my scenario:

+ (UIColor *)ivl_randomColorWithSeedString:(NSString *)seedString {

    srand48(seedString ? seedString.hash : arc4random());

    float red = 0.0;
    while (red < 0.1 || red > 0.84) {
        red = drand48();
    }

    float green = 0.0;
    while (green < 0.1 || green > 0.84) {
        green = drand48();
    }

    float blue = 0.0;
    while (blue < 0.1 || blue > 0.84) {
        blue = drand48();
    }

    return [UIColor colorWithRed:red green:green blue:blue alpha:1.0f];
}

what I'd like to achieve is getting a random color for a different string, if I have the same string then return the same index.

- (void)setImageWithImage:(UIImage *)image orString:(NSString *)string colorArray:(NSArray *)colorArray circular:(BOOL)isCircular textAttributes:(NSDictionary *)textAttributes {
  if (!textAttributes) {
    textAttributes = @{
                       NSFontAttributeName: [self fontForFontName:nil],
                       NSForegroundColorAttributeName: [UIColor whiteColor]
                       };
  }

  NSMutableString *displayString = [NSMutableString stringWithString:@""];

  NSMutableArray *words = [[string componentsSeparatedByCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]] mutableCopy];

  //
  // Get first letter of the first and last word
  //
  if ([words count]) {
    NSString *firstWord = [words firstObject];
    if ([firstWord length]) {
      // Get character range to handle emoji (emojis consist of 2 characters in sequence)
      NSRange firstLetterRange = [firstWord rangeOfComposedCharacterSequencesForRange:NSMakeRange(0, 1)];
      [displayString appendString:[firstWord substringWithRange:firstLetterRange]];
    }

    if ([words count] >= 2) {
      NSString *lastWord = [words lastObject];

      while ([lastWord length] == 0 && [words count] >= 2) {
        [words removeLastObject];
        lastWord = [words lastObject];
      }

      if ([words count] > 1) {
        // Get character range to handle emoji (emojis consist of 2 characters in sequence)
        NSRange lastLetterRange = [lastWord rangeOfComposedCharacterSequencesForRange:NSMakeRange(0, 1)];
        [displayString appendString:[lastWord substringWithRange:lastLetterRange]];
      }
    }
  }

  srand48(string ? string.hash : arc4random());

  NSUInteger randomIndex = arc4random() % [colorArray count];
  UIColor *backgroundColor = colorArray[randomIndex];

  if (image != nil) {
    self.image = image;
  } else {
    self.image = [self imageSnapshotFromText:[displayString uppercaseString] backgroundColor:backgroundColor circular:isCircular textAttributes:textAttributes];
  }
}

What's the best approach on this?