I have created two horde bug reports, if you want to follow them, they are:
http://bugs.horde.org/ticket/?id=4423
I have good news and bad news. This was a bear to figure out until I installed phpmyadmin.
The good news, here is the format the values need to be in.
// categories
$_prefs['categories'] = array(
'value' => "Meeting|Lunch|Vacation",
'locked' => false,
'shared' => true,
'type' => 'implicit'
);
// category colors
$_prefs['category_colors'] = array(
'value' => "Meeting:#AABBCC|Lunch:#BBDDCC|Vacation:#EEFFFF",
'locked' => false,
'shared' => true,
'type' => 'implicit'
);
Now for the bad news, the users still have the option to change the colors. If you set the categories to locked, then users can't create any categories. If you set category_colors to locked, then all categories are saved as the default color.
So, here is better news, but it will take some extra work to get accomplished. The default and unfiled categories are defined in these two files:
/home/httpd/html/horde/lib/Horde/Prefs/CategoryManager.php
/home/httpd/html/horde/templates/prefs/categorymanagement.inc
You can modify those files to add your own defaults, plus you can also modify them so that your default settings can not be changed. The only thing you will have to remember is that if/when there is an update to horde, you will lose those settings, so back those files up.
John Bennett