http://www.americancatholic.org/messenger/Mar2002/Wiseman.asp
Q: Last year on Palm Sunday they read two Gospel passages: Jesus' entry into Jerusalem and then the Passion. As a child, I remember only the entry into Jerusalem story being read on Palm Sunday. Did the Church move the Passion account to Sunday to make sure more people heard it?
Also, when Jesus comes into Jerusalem everyone cheers. Five days later they call for his execution. That is a big turnaround in only five days! Did throwing the moneychangers out of the Temple account for that change?
Finally, why is red the liturgical color for Palm Sunday?
A: There is a Passion account in each Gospel. For centuries, the Gospel of John's account was read on Good Friday and the Gospel of Matthew was read on Palm Sunday (Passion Sunday now). So that all accounts are read, since 1969 the Catholic Church reads on Palm Sunday in rotation: Matthew (Year A—2002), Mark (Year B) and Luke (Year C). The accounts of Jesus' triumphal entry into Jerusalem are read in the same rotation.
The question of the crowd's turnaround from Palm Sunday to Good Friday is an important one. The Gospels may have created the impression in our minds that everyone in Jerusalem hailed Jesus as Messiah on Palm Sunday and that by Good Friday everyone in Jerusalem sought Jesus' death. In fact, neither statement is true.
Matthew 26:5 says that the Jewish leaders feared a riot among the people if Jesus was seized during the Passover festival. Mark 15:11 says that the chief priests "stirred up the crowd to have him [Pilate] release Barabbas for them instead." That crowd did not speak for all Jewish people in Jerusalem. Luke 23:50 says that, although he was a member of the Sanhedrin, Joseph of Arimathea did not consent to their plan. In fact, the Palm Sunday/Good Friday time span does not represent a shift from total acceptance of Jesus to total rejection of him.
Red is used liturgically on Palm Sunday and on Good Friday because that is the color for martyrs. These two feasts mark the last days of Jesus, the innocent one who died for the guilty.