To fill any color above the background image select the container containing the image, go to the Widget Properties Panel/Background/Overlay color and choose the color and transparency!
Color overlay
Background fill
To fill a background color to whole project page go to Properties Panel/Page/Appearance and type in a color into the Background input field
To Add one color background to your widget go to Widget Properties Panel/Background
To add image as a background go to Widget Properties Panel/Background and press the Add image button:
or double-click on the Widget.