some folders yellow, some blue

Does anyone know why?

Seems that folders excluded from tagging are shown in blue while others are shown in yellow. New feature in the 2.0 release.

@redskinsfan is correct. If you don’t care for the mixed group colors, set the switch for ‘Preferences>General>Colorize icons with label’ and the yellow groups (groups not excluded from tagging) will also show in blue, with small icon tags to the right of the group names to indicate that the groups have tagging enabled.

Thanks. Now that I know what it means, I enjoy the feature.

What I don’t get is why some groups are exclude from tagging in the first place. And from classification? Is that do that the database doesn’t get bogged down by superfluous items in the database?

Yes, that’s one good reason. Another is that some groups – those with RSS feeds in them – can get really clogged with tags generated by the feed. Another reason is that some users don’t use tagging or want to restrict its scope. Same for classification. Being able to turn these features off and on merely adds flexibility for a broader user population.