Copying

If you want to change the colour scheme for an entire presentation it is best to make your changes to the actual slide master.
However if you only want to make changed to several slide it is possible to make the changes to one of the slides and then copy the colour scheme across to your other slides.
Display your presentation in Slide Sorter view.
Select the slide containing the colour scheme you want to copy.
Click the Format Painter button on the formatting toolbar.
Click on the slides you want to copy the colour scheme to.
This also allows to copy backgrounds from one presentation to another.