The coloured dots represent the online status of your matches. Depending on their last login, the little circle will be green, blue or grey.
Green indicates that this member logged into their account in the last 24 hours.
A blue dot indicates that this match has logged on at some point in the past 28 days.
A grey dot next to the your match’s name ("last logged in over a week ago") means that the user has not logged in to their account in over 28 days. They might be on holiday, for example.