How to get pixel color from SKSpriteNode or from SKTexture?

In Sprite Kit you don't have access to texture data.

Instead you'd have to create a bit mask from the image, for example by loading the image as UIImage or CGImage and scanning for all non-transparent pixels.

You can subclass SKSpriteNode, add a reference to the image and check the alpha of that image, when detecting touches.

1) Add an instance variable to hold your image

@implementation DraggableSprite {
    UIImage *_holdingImage;
}

2) Then in init method you can save the reference to that image

-(id)initWithImageNamed:(NSString *)name {
    if (self=[super initWithImageNamed:name]) {

        self.userInteractionEnabled = YES;//enable touches handling
        self.anchorPoint = P(0,0);
        _holdingImage = [UIImage imageNamed:name];
    }
    return self;
}

3) Detect the touches

-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    UITouch *touch = [touches anyObject];
    CGPoint positionInScene = [touch locationInNode:self];

    CGFloat a = [UIImage getAlphaFromImage:_holdingImage atX:positionInScene.x andY:self.size.height-positionInScene.y];
    NSLog(@"alpha is %f",a);
}

4) The alpha checking function

+(CGFloat)getAlphaFromImage:(UIImage*)image atX:(NSInteger)xx andY:(NSInteger)yy {
    // Cancel if point is outside image coordinates
    if (!CGRectContainsPoint(CGRectMake(0.0f, 0.0f, image.size.width, image.size.height), CGPointMake(xx,yy))) {
        return 0;
    }

    // Create a 1x1 pixel byte array and bitmap context to draw the pixel into.
    // Reference: http://stackoverflow.com/questions/1042830/retrieving-a-pixel-alpha-value-for-a-uiimage
    NSInteger pointX = xx;
    NSInteger pointY = yy;
    CGImageRef cgImage = image.CGImage;
    NSUInteger width = image.size.width;
    NSUInteger height = image.size.height;
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    int bytesPerPixel = 4;
    int bytesPerRow = bytesPerPixel * 1;
    NSUInteger bitsPerComponent = 8;
    unsigned char pixelData[4] = { 0, 0, 0, 0 };
    CGContextRef context = CGBitmapContextCreate(pixelData,
                                                 1,
                                                 1,
                                                 bitsPerComponent,
                                                 bytesPerRow,
                                                 colorSpace,
                                                 kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
    CGColorSpaceRelease(colorSpace);
    CGContextSetBlendMode(context, kCGBlendModeCopy);

    // Draw the pixel we are interested in onto the bitmap context
    CGContextTranslateCTM(context, -pointX, pointY-(CGFloat)height);
    CGContextDrawImage(context, CGRectMake(0.0f, 0.0f, (CGFloat)width, (CGFloat)height), cgImage);
    CGContextRelease(context);

    CGFloat alpha = (CGFloat)pixelData[3] / 255.0f;
    return alpha;
}

Here is how to get the color of one pixel directly from the SKTexture:

class SpriteKitUtility
{
    class func pixelFromTexture(texture: SKTexture, position: CGPoint) -> SKColor {
        let view = SKView(frame: CGRectMake(0, 0, 1, 1))
        let scene = SKScene(size: CGSize(width: 1, height: 1))
        let sprite  = SKSpriteNode(texture: texture)
        sprite.anchorPoint = CGPointZero
        sprite.position = CGPoint(x: -floor(position.x), y: -floor(position.y))
        scene.anchorPoint = CGPointZero
        scene.addChild(sprite)
        view.presentScene(scene)
        var pixel: [Byte] = [0, 0, 0, 0]
        let bitmapInfo = CGBitmapInfo(CGImageAlphaInfo.PremultipliedLast.rawValue)
        let context = CGBitmapContextCreate(&pixel, 1, 1, 8, 4, CGColorSpaceCreateDeviceRGB(), bitmapInfo);
        UIGraphicsPushContext(context);
        view.drawViewHierarchyInRect(view.bounds, afterScreenUpdates: true)
        UIGraphicsPopContext()
        return SKColor(red: CGFloat(pixel[0]) / 255.0, green: CGFloat(pixel[1]) / 255.0, blue: CGFloat(pixel[2]) / 255.0, alpha: CGFloat(pixel[3]) / 255.0)
    }
}