According to the first WYD2011 Press Conference overnight, the graphic designer, Jose Gil-Nogues Vila explained:

• Cross obviously represents Christianity and relates to the WYD theme: "rooted and built up in Christ, strong in faith".
• The crown is actually formed by 3 people holding hands, if you look very closely - representing young people of the world coming together to celebrate their faith with the Holy Father "at the foot of the cross".  The formed crown also represents the Virgen de la Almudena, patron saint of Madrid.  Crown also stresses a "M" for Mary and also "M" for Madrid... how clever!
• Apparently, the 3 colours red, orange and yellow represents the heat of Madrid in August... and also the warmth of God as "Trinity of Love"...  Although not explained in the press release, but I have noticed that red and yellow are also the colours of the Spanish flag... what a funny coincidence!