Trello – I can’t add stickers anymore to the Trello board

I'm not sure if this is the issue that you're facing, but there is a bug in Trello that prevents people from accepting invitations to public boards.

(The fix for that will go out with the next release of Trello)

You can give the coloured labels meaningful names, which I've found really helps.

It's not entirely obvious how to do this, at first:

1. When viewing a board, click on the grey Icon just to the left of the board's name
2. Select "Preferences"
3. You'll see the labels with names like "Green (default)"
4. Click on the name of a label, a type in something meaningful, and save it.

Now, when you click on a card, and then "Labels...", the name you specified is displayed on top of the Colour swatch.

This at least saves people from having to remember the meaning you've ascribed to the different colours.

You still only get 6 colours to play with, though, and I don't believe that you can change the colours themselves.

Other nice tricks

1. If you are hovering over a card, and hit the letter L on the keyboard, it pops up the list Labels, which is rather convenient.
2. The "Filter Cards..." option at top-right allows you to use these labels for filtering.

Edit 2011-11-19

I've just noticed that there's now a shorter route to renaming labels: At the bottom of the list of labels, there is now a link to "Edit labels (sic) titles":