Let's render an image as an example:

Now, save the rendered image as a JPG file (called "original.jpg"), and drag it to the Compositing mode, and connect to the Composite node, then save the newly composited image as "new.jpg".
The following is the comparison between the "orginal.jpg" VS "new.jpg".
The brightness or the contrast is changed - "new.jpg" becomes darker and less saturated.
(1) Why is this happening please?
(2) How to avoid or fix this change please?
For those who wonder why I do this is because I want to composite two images together without affecting the brightness or contrast or saturation. For example, let's compose it with an image of red square:
Again, the brightness or contrast or saturation of the original image is changed after this composition.





