Why would more countries join the communist side if colonialism had continued during the Cold War?

First, for oppressed people, Communism sounds rather appealing.  The people get rid of their upper-class oppressors, and many in the developing world felt this way towards the Western colonial powers.  However, these people were not true Marxists.  The Soviet Union, in order to gain support overseas, sent aid and military advisers to countries that they thought would be friendly to them.  North Vietnam asked for and received lots of support from the Soviet Union during the Vietnam War--Hanoi's air defense system was largely funded by Moscow.  Of course, the developing world was often able to play the rival superpowers off each other.  The United States often had to match and exceed the aid package given by the Soviet Union in order to make sure that its potential client states stayed in the Western camp.  

You mention colonialism continuing after the Cold War, and outside of monetary and ideological reasons, this would have been very difficult to continue in the traditional sense anyway.  When you look at the traditional colonial powers of Europe, France and Britain, they payed a very heavy price in terms of blood and treasure during WWII.  For a time during the war, France lost its colonial possessions to the Axis powers.  WWII was the final death blow against colonialism for the Western world and the end of this time period began the rush to create client states by the two remaining superpowers, America and the Soviet Union.