A diagram can have a background colour or a picture.
The colour is set in Page Details.
You can paste a picture into a diagram background using Paste Special
Another way is in Diagram Details form, where you can select a picture file to use as the background for a diagram. This means that you can now use project P&ID's as background. To do this, use your P&ID software to export a picture as a Windows Picture file (emf, gif,bmp, jpg are supported) to and then point to the file from page details. If the picture has been changed ControlDraw will produce a message when you open the page, and record the changes in the page history. Also the page keeps a local copy of the picture so that if the picture file is removed the background is retained.
You can switch the background picture on and off with the right mouse butten and clear the picture from the Diagram Details form
See also Draw using a P&ID