If the palette is only black and write, try to kill your browser or the other application which could take all colors available. Places become definitive after 5 minutes. This action opens the message window. Click in the corresponding global image on a participant place and look at the 'Send message to'. Write a small text in the 'Send message to' window followed by 'return'.
Participants receives message in the 'Message from' window.
This is an easy way to communicate with the whole image. You can identify correspondants by their names or alias. A window displays the list of connected participants. Simply click on the one you want. The 'Anonymous' and 'Listen to the whole image' do not work.