I second @Herbie that the CMYK conversion is a good idea towards the extraction. The result of this conversion is not a color image anymore. If you look at the top of the image it actually will state 32-bit, refering to a grayscale image holding only intensity values (not color). In such cases you will need to apply a “normal” intensity threshold. I would suggest, that you first get familiar with the topics on image segmentation and thresholding. There are numerous great resources on the ImageJ page. Here an excerpt:
Thus, you could try starting e.g. from the magenta channel of the CMYK color space or similarly convert your image into LAB and look into the L* and the a* channel and try the thresholding there.
Alternatively, for the beginning it might be easier to go via the Trainable WEKA Segmentation. See the following test segmentation on the image you provided:
The corresponding colored areas you can extract for example by clicking on “Get Result” and set a manual threshold to the respective index of the colored output (you see this by hovering your mouse over the color in the output image and checking the ImageJ main window. This will display the index of the respective color).
The remaining small non-specific areas could for example be eliminated by size and shape exclusion using the ►Analyze ►Analyze Particles… function in ImageJ.
Alternatively, you can then threshold the outer part (in the upper image shown in green) and take this as sample area to calculate e.g. the %Area of the reddish staining (if this is what you need as an output.
Those steps can also be recorded with the macro recorder (►Plugins ►Macros ►Record…)
Hope this helps you for getting started