Measuring Grey Intensity

Hello forum,

Here is my ultimate goal:  I have an image that I want to be able to quantify the amount or intensity of purple on the image.  There are other colors present and there are different shades or purple throughout the image.

I was able to isolate just the purple on the image but going to

Select>Color Range>

Then selecting only the purple on the image-- it changed it from a colored image to now greyscale-- where the darkest shades of purple are dark grey vs little amounts of purple which are a lighter grey.

Now I want to be able to quantify the intensity of the grey on the ENTIRE image (more specifically the entire dish in the image)

Is there any way I can do this?  I have Photoshop CS6.  Also is there a specific format my image needs to be in order to start analyzing-- I found a great approach that I could use already (found here Photoshop Extended: Performing a comparative analysis of bands | lynda.com - YouTube))

https://www.youtube.com/watch?v=v5xLSXMHFr0however whenever I go to

Image>Analysis>Set Measurements Scale (or any other selection in within that tab)  it remains grey meaning I cannot select that item.  The only team I can select is ruler tool which is not what I want.  Here is an image to show you more clearly what I'm talking about (the background is the image I want to analyze)

I hope this makes sense-- let me know if you need clarification.