Hi Frode,

which version of PowerPoint do you use? Up to PowerPoint 2003 the number of
colors is limited to 8, in PowerPoint 2007 you can set 10 colors (plus 2 for
Hyperlinks). There is no chance to change this to 15. But in PowerPoint 2007
each of the 10 colors has 5 lighter and darker shades set automatically.

Best regards,
Ute

In PowerPoint 2002/2003 you can customize your color palette in Format >
Slide Design > Edit color palette. Since the number of colors you can
customize is limited, you may want to set up two or three schemes: Primary,
Secondary and Tertiary. You'll want to keep text, background and shadows the
same for all schemes.

Delete all other schemes from the palette.

Please note that establishing these scheme will not prevent users from
creating their own colors.

To go along with the good advice Ute and Sandy have provided, this page may
help you better understand how color schemes work:
http://www.echosvoice.com/colorschemes.htm

With PowerPoint 2007, you can also add custom colors to the
fill/outline/font color drop-downs (they show up in a section called,
appropriately enough, "custom colors") by editing the XML of the theme or
template. This might be a way to add the 15 colors and have them always
available, even if they don't affect the various "quick styles" that pick up
their settings from the actual theme color scheme.