I looked at both posts and have a few suggestions and comments.
I do not understand why you need to desaturate the images in Photoshop first. It is not usually very useful to work in Photoshop as you can probably do everything in Fiji.
Your question is actually two questions that need to be tackled separately. First, we figure how to find the area to quantify, and in a second step, we see how we can measure the ‘redness’ of that area.
Let’s ignore the area measurement and focus on the red part. I am under the impression that you had the answer already when you were talking about ‘desaturating’ the images. A good measure of purity of redness is, exactly the ‘saturation’ of the image.
Fig 2 seems to have a red that is about a quarter of the red in Fig 1.
This can be achieved by converting the image to HSB in Fiji (Image > Type > HSB Stack) and measuring the mean of second channel.
Now onto area measurement.
It is not a good strategy to detect and measure something based on the same signal. What if there is no red? How do you define the area then? The idea is to use something independent to find the area to measure, like the outline of the leg. A good trick might be to ensure the pictures are taken in a consistent way so as to stack them together, almost aligned. Then measure the saturation in the image.
To select only the red component, you could use the Hue component in the HSB image to limit the saturation measurement to only the ‘red’ areas.