A bunch of objective-c color helpers

ColorHelper.h

#import <Foundation/Foundation.h>
#import <QuartzCore/QuartzCore.h>

// generate a UIColor from rgb and alpha values
#define RGBA(r, g, b, a) [UIColor colorWithRed:r/255.0 green:g/255.0 blue:b/255.0 alpha:a]
#define RGB(r, g, b) RGBA(r, g, b, 1.0)
// generate a UIColor
#define GRAYSCALEA(rgb, a) RGBA(rgb, rgb, rgb, a)
#define GRAYSCALE(rgb) GRAYSCALEA(rgb, 1.0)
// generate a UIColor from a hex and an alpha value
#define HEXCOLORA(rgbValue, a) [UIColor colorWithRed:((float)((rgbValue & 0xFF0000) >> 16)) / 255.0 green:((float)((rgbValue & 0xFF00) >> 8)) / 255.0 blue:((float)(rgbValue & 0xFF)) / 255.0 alpha:a]
#define HEXCOLOR(rgbValue) HEXCOLORA(rgbValue, 1.0)

@interface ColorHelper : NSObject

// UImage of a solid color and size. Good for placeholders.
+ (UIImage *)imageFromColor:(UIColor *)color andSize:(CGSize)size;
// vertical linear gradient at the given frame from topColor to bottomColor
+ (CAGradientLayer *)gradientWithFrame:(CGRect)frame fromColor:(UIColor *)topColor toColor:(UIColor *)bottomColor;
// darkens a UIColor by a given amount
+ (UIColor *)darkenColor:(UIColor *)oldColor percentOfOriginal:(float)amount;
// lightens a UIColor by a given amount
+ (UIColor *)lightenColor:(UIColor *)oldColor byPercentage:(float)amount;

@end

ColorHelper.m

@implementation ColorHelper

+ (UIImage *)imageFromColor:(UIColor *)color andSize:(CGSize)size
{
    CGRect rect = CGRectMake(0, 0, size.width, size.height);
    UIGraphicsBeginImageContext(rect.size);
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSetFillColorWithColor(context, [color CGColor]);
    CGContextFillRect(context, rect);
    UIImage *img = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    return img;
}

+ (CAGradientLayer *)gradientWithFrame:(CGRect)frame fromColor:(UIColor *)topColor toColor:(UIColor *)bottomColor
{
    CAGradientLayer *shadowGradient = [CAGradientLayer layer];
    shadowGradient.frame = frame;
    shadowGradient.colors = [NSArray arrayWithObjects:(id)[topColor CGColor], (id)[bottomColor CGColor], nil];
    return shadowGradient;
}

+ (UIColor *)darkenColor:(UIColor *)oldColor percentOfOriginal:(float)amount
{
    float percentage      = amount / 100.0;
    int   totalComponents = CGColorGetNumberOfComponents(oldColor.CGColor);
    bool  isGreyscale     = totalComponents == 2 ? YES : NO;
    
    CGFloat* oldComponents = (CGFloat *)CGColorGetComponents(oldColor.CGColor);
    CGFloat newComponents[4];
    
    if (isGreyscale) {
        newComponents[0] = oldComponents[0]*percentage;
        newComponents[1] = oldComponents[0]*percentage;
        newComponents[2] = oldComponents[0]*percentage;
        newComponents[3] = oldComponents[1];        
    } else {
        newComponents[0] = oldComponents[0]*percentage;
        newComponents[1] = oldComponents[1]*percentage;
        newComponents[2] = oldComponents[2]*percentage;
        newComponents[3] = oldComponents[3];        
    }
    
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
	CGColorRef newColor = CGColorCreate(colorSpace, newComponents);
	CGColorSpaceRelease(colorSpace);
    
	UIColor *retColor = [UIColor colorWithCGColor:newColor];
	CGColorRelease(newColor);
    
    return retColor;
}

+ (UIColor *)lightenColor:(UIColor *)oldColor byPercentage:(float)amount
{
    float percentage      = amount / 100.0;
    int   totalComponents = CGColorGetNumberOfComponents(oldColor.CGColor);
    bool  isGreyscale     = totalComponents == 2 ? YES : NO;
    
    CGFloat* oldComponents = (CGFloat *)CGColorGetComponents(oldColor.CGColor);
    CGFloat newComponents[4];
    
    // FIXME: Clean this SHITE up
    if (isGreyscale) {
        newComponents[0] = oldComponents[0]*percentage + oldComponents[0] > 1.0 ? 1.0 : oldComponents[0]*percentage + oldComponents[0];
        newComponents[1] = oldComponents[0]*percentage + oldComponents[0] > 1.0 ? 1.0 : oldComponents[0]*percentage + oldComponents[0];
        newComponents[2] = oldComponents[0]*percentage + oldComponents[0] > 1.0 ? 1.0 : oldComponents[0]*percentage + oldComponents[0];
        newComponents[3] = oldComponents[0]*percentage + oldComponents[1] > 1.0 ? 1.0 : oldComponents[1]*percentage + oldComponents[1];
    } else {
        newComponents[0] = oldComponents[0]*percentage + oldComponents[0] > 1.0 ? 1.0 : oldComponents[0]*percentage + oldComponents[0];
        newComponents[1] = oldComponents[1]*percentage + oldComponents[1] > 1.0 ? 1.0 : oldComponents[0]*percentage + oldComponents[1];
        newComponents[2] = oldComponents[2]*percentage + oldComponents[2] > 1.0 ? 1.0 : oldComponents[0]*percentage + oldComponents[2];
        newComponents[3] = oldComponents[3]*percentage + oldComponents[3] > 1.0 ? 1.0 : oldComponents[0]*percentage + oldComponents[3];
    }
    
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
	CGColorRef newColor = CGColorCreate(colorSpace, newComponents);
	CGColorSpaceRelease(colorSpace);
    
	UIColor *retColor = [UIColor colorWithCGColor:newColor];
	CGColorRelease(newColor);
    
    return retColor;
}
@end

Thanks for sharing, thanks @bartjacobs for the great comment.

I would add this to this bunch of helpers:

+ (UIColor *)colorWithHexColorString:(NSString*)hexColorString
{
  UIColor* result = nil;
  unsigned colorCode = 0;
  unsigned char redByte, greenByte, blueByte;

  if (nil != hexColorString)
  {
    NSScanner* scanner = [NSScanner scannerWithString:hexColorString];
    (void) [scanner scanHexInt:&colorCode]; // ignore error
  }
  redByte = (unsigned char)(colorCode >> 16);
  greenByte = (unsigned char)(colorCode >> 8);
  blueByte = (unsigned char)(colorCode); // masks off high bits

  result = [UIColor colorWithRed:(CGFloat)redByte / 0xff
                           green:(CGFloat)greenByte / 0xff
                            blue:(CGFloat)blueByte / 0xff
                           alpha:1.0];
  return result;
}

I took it from a StackOverflow question I think, but I haven't the source anymore. For the author, just comment and post the link ;)