I have an image with a "rainbow" (a gradient of 7 colors).
How can I select each and every pixel of that image that contains at least a shade of red? That is, every color whose Red component (from its RGB code) is between 1 and 255.
Can you give a little more detail as what you are intending to do? If you are wanting to SELECT pixels containing red, you are creating a mask. Thinking of the mask in terms of greyscale...do you want all pixels containing red to appear as white on the mask (100% selected or opaque) or do you want the mask to reflect the percentage of red contained in the pixel? i.e. pure red will be pure white on the mask. A pixel containing no red at all will be black on the mask. A pixel which has a red value of 127 would appear as 50% grey on the mask, etc.
I am not sure what you are trying to do.