如图所示：

image.png

实现将橙、红两色从左到右渐变，生成一张尺寸为(200,100)的图片。
调用示例为：

UIImage *image = [UIImage imageFromGradientColors:@[UIColor.redColor,UIColor.orangeColor] gradientType:GradientTypeLeftToRight imageSize:CGSizeMake(200, 100)];
    self.imageView.image = image;

分类实现完整代码为:
UIImage+newImage.h

#import <UIKit/UIKit.h>
typedef NS_ENUM(NSUInteger, GradientType) {
    
    GradientTypeTopToBottom      = 0,//从上到小
    GradientTypeLeftToRight      = 1,//从左到右
    GradientTypeUpleftToLowright = 2,//左上到右下
    GradientTypeUprightToLowleft = 3,//右上到左下
};
NS_ASSUME_NONNULL_BEGIN

@interface UIImage (newImage)

/**
 通过渐变色生成图片
 
 @param colors 渐变颜色数组
 @param gradientType 渐变类型
 @param imageSize 需要的图片尺寸
 
 */
+ (UIImage *)imageFromGradientColors:(NSArray *)colors gradientType:(GradientType)gradientType imageSize:(CGSize)imageSize;

@end

NS_ASSUME_NONNULL_END

UIImage+newImage.m

#import "UIImage+newImage.h"

@implementation UIImage (newImage)

+ (UIImage *)imageFromGradientColors:(NSArray *)colors gradientType:(GradientType)gradientType imageSize:(CGSize)imageSize {
    
    NSMutableArray *array = [NSMutableArray array];
    for(UIColor *color in colors) {
        [array addObject:(id)color.CGColor];
    }
    array = (NSMutableArray *)[[array reverseObjectEnumerator] allObjects];
    UIGraphicsBeginImageContextWithOptions(imageSize, YES, 1);
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSaveGState(context);
    //    CGColorSpaceRef colorSpace = CGColorGetColorSpace([[colors lastObject] CGColor]);
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    CGGradientRef gradient = CGGradientCreateWithColors(colorSpace, (CFArrayRef)array, NULL);
    CGPoint start;
    CGPoint end;
    switch (gradientType) {
        case GradientTypeTopToBottom:
            start = CGPointMake(0.0, 0.0);
            end = CGPointMake(0.0, imageSize.height);
            break;
        case GradientTypeLeftToRight:
            start = CGPointMake(0.0, 0.0);
            end = CGPointMake(imageSize.width, 0.0);
            break;
        case GradientTypeUpleftToLowright:
            start = CGPointMake(0.0, 0.0);
            end = CGPointMake(imageSize.width, imageSize.height);
            break;
        case GradientTypeUprightToLowleft:
            start = CGPointMake(imageSize.width, 0.0);
            end = CGPointMake(0.0, imageSize.height);
            break;
        default:
            break;
    }
    CGContextScaleCTM(context,1.0, -1.0);
    CGContextTranslateCTM(context,0, -imageSize.height);
    CGContextDrawLinearGradient(context, gradient, start, end, 0);
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    CGGradientRelease(gradient);
    CGContextRestoreGState(context);
    CGColorSpaceRelease(colorSpace);
    UIGraphicsEndImageContext();
    return image;
}

@end

指定单一颜色，生成图片:

+ (UIImage *)imageWithColor:(UIColor *)color size:(CGSize)size {
    CGRect rect = CGRectMake(0, 0, size.width, size.height);
    
    UIGraphicsBeginImageContext(size);
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSetFillColorWithColor(context, color.CGColor);
    CGContextFillRect(context, rect);
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    
    return image;
}