Can a colour be added as a background on a blank slide

You’ve got two options: 1) In the main menu, select Slideshow > Settings… > Settings (tab) > then choose a Background color from the dialog box. Be aware that the background color applies to ALL of the slides in your presentation (that may or may not be a drawback, depending on your needs) or 2) In Photoshop or any other “paint” program create a solid rectangle in the color you want and save it in .jpg format (large enough to cover the working stage area). If it’s a little too big, that’s okay. Save it wherever, then drag it in as the bottom image for any slide you want it applied to. The advantage is that you can create multiple color files and use whichever you want for any slide (green on one, red on another, etc.). The disadvantage is that you are using up ONE layer on the slide – and you’ve only got SIX total to play with.