the slide presentation is 40 slides
white background
black text
I want to put some colour
and add some images
Use View>Slide Master. Then from the context menu of the slide use the item Format Slide to set a background color (and other things), or Insert Image for an image that you will show on each slide, or Set Background Image for an image, that is behind all other content of the slide master. You can add text, which is to repeated on each slide here too, using a shape.
Make sure you click on an empty part of the slide and not on one of the placeholders, otherwise you will format the presentation objects and not the slide background.