How to get color components of a CGColor correctly

Most likely wrong colorspace. I guess the alpha component of the grayscale colorspace gets where you think the green component should be.
I use this function to create a string from UIColor, I only encounter RGB and Grayscale colorspaces, so I just interpret every color with less than 4 (R+G+B+A) components as grayscale.

if (CGColorGetNumberOfComponents(color.CGColor) < 4) {
    const CGFloat *components = CGColorGetComponents(color.CGColor);
    color = [UIColor colorWithRed:components[0] green:components[0] blue:components[0] alpha:components[1]];
}
if (CGColorSpaceGetModel(CGColorGetColorSpace(color.CGColor)) != kCGColorSpaceModelRGB) {
    NSLog(@"no rgb colorspace");
    // do seomthing
}
const CGFloat *components = CGColorGetComponents(color.CGColor);
NSString *colorAsString = [NSString stringWithFormat:@"%f,%f,%f,%f", components[0], components[1], components[2], components[3]];

of course this method is not save for all cases, you should adopt it to your requirements.

The reason you are seeing what looks like r=0, g=1, b=0, a=0 is because you are misinterpreting the values in the returned array as being in an RGB color model. UIColor uses monochrome colorspace for greyscale colors like black in this case.
What you are seeing is an array of 2 components from a monochrome color model. The first is gray level (0 for black) and the second is alpha (1 for opaque). The last two values your are looking at are off the end of the 2 element array and happen to be 0 in this case.

You'll notice if color is black and you try CGColorGetNumberOfComponents(color.CGColor), it returns 2. And if you try CGColorSpaceGetModel(CGColorGetColorSpace(color.CGColor)), it returns 0 which corresponds to kCGColorSpaceModelMonochrome (see the enum CGColorSpaceModel in CGColorSpace.h)

see CGColorSpace Reference

I think this is a very nice way to get the rgb representation of any UIColor*, which already has a convenience method for retaining it´s components.

-(CGColorRef)CGColorRefFromUIColor:(UIColor*)newColor {
     CGFloat components[4] = {0.0,0.0,0.0,0.0};
     [newColor getRed:&components[0] green:&components[1] blue:&components[2]        alpha:&components[3]];
     CGColorRef newRGB = CGColorCreate(CGColorSpaceCreateDeviceRGB(), components);
     return newRGB;
}