Once you've used additional colors, they stay on the palette, up to 8 of them.
Once you choose 8, then as you choose more colors, the new ones replace the
earlier ones.  If you just need to avoid choosing the wrong color by mistake,
draw a rectangle, use the fill command and pick More Colors; choose white or
black.  Do that again but this time change the RGB values on custom colors so
that it's only one number different from pure white or black.  Repeat until
you've got all white plus the one color you want.  

Or try this little macro that'll automate it for you:

Sub LigthtenItUp()

   ActivePresentation.ExtraColors.Add RGB(255, 255, 255)
   ActivePresentation.ExtraColors.Add RGB(255, 255, 254)
   ActivePresentation.ExtraColors.Add RGB(255, 254, 255)
   ActivePresentation.ExtraColors.Add RGB(254, 255, 255)
   
   ActivePresentation.ExtraColors.Add RGB(255, 254, 254)
   ActivePresentation.ExtraColors.Add RGB(254, 254, 255)
   ActivePresentation.ExtraColors.Add RGB(254, 254, 254)
   ' and change this to the RGB values of the color you really want
   ActivePresentation.ExtraColors.Add RGB(123, 0, 123)
   
End Sub

If you don't know what to do with the code above, see:

How do I use VBA code in PowerPoint?
http://www.pptfaq.com/FAQ00033.htm

-----------------------------------------
Steve Rindsberg, PPT MVP
PPT FAQ:  www.pptfaq.com
PPTools:  www.pptools.com
================================================

They can also be deleted in the script editor
shift alt f11 to get there
View Project explorer and open pres.xml

Search for lines that start with <o:colormru v:ext="edit"
The extra colors are listed thus:
eg  <o:colormru v:ext="edit" colors="#f83b1c"/> where #f83b1c is the hex
colour
Change to <o:colormru v:ext="edit" colors=""/> to delete and return to
powerpoint

Amazing PPT Hints, Tips and Tutorials-http://www.PPTAlchemy.co.uk
http://www.technologytrish.co.uk/ppttipshome.html
email john AT technologytrish.co.uk