New Post: cmyk image with only tones of black color

I want to draw rectangles and texts with Magick.net lib over a cmyk image which has only black color values. I don't want cyan,magenta and yellow color values on final image per black only printing requirement. I tried draw function but could not eliminate cmy values.
I have some results with Composite() and CompositeOperator but i want to do it right away.
Thanks in advance