First regarding screenshots:
Secondly you may be able to use the Histogram, automating that might take some Scripting.
Then selecting only the purple on the image-- it changed it from a colored image to now greyscale
Which exact steps are you describing here?
Here is my original image. On the top selection bar I clicked on
Then this little window pops up (shown up above)
Right below the save button on the window there are "eyedroppers" that allow you to select the color you want to isolate. I isolated the purple hues and now the image is greyscale
The darkest grey areas are the most concentration of purple where the light grey areas are where no purple is found.
But how and why did you arrive at a grayscale image after selecting the Colour Range?
Please post proper screenshots in future, not photographs of your screen.
In the pop up box, there is a box that says selection preview and it can convert it into greyscale.
I converted it to greyscale because I thought it would be easier for a program to measure grey intensity versus intensity of all other colors on the image.
Check out the Histogram.
Have you posted on the Scripting Forum yet?