You can do number of ways: (Considering the single pixel value, as both section has same pixel value, may have small different)
Move the cursor, where you need the pixel value, it shows the image viewer at the bottom of the app. Note this and find the difference.
2. Authomatically: Using code (Assumed that your contours cover the mid section of the image)
[r c d]=size(data);
Kalyan Acharjya 😊