When you decline, it's red. For a challenge you issued, the message appears in blue if you choose to cancel it. On the news log, entries for games won appears in green. Games forfieted are red. Challenges declined are purple.
Challenges accepted are blue.
New player signups appear in yellow. Cancelled games are cyan. The names of players who are currently online with you will appear in bold print most places. Those who aren't online appear normal. Gray arrows on the right mean you're already at the end.