To add a color or image overlay to the background of a section in Elementor, you should create a section with a background and follow the steps below:
1. Select the section to which you want to add an overlay in Elementor. By doing so, a new dashboard on the left with element's settings will pop up.
2. Then, in the Style tab, open the Background Overlay accordion panel and select a color or an image for it.
3. Publish the changes.
Note: You can also add a background overlay to a column by following the guidelines described above.