The red and orange routes cover the district of Ciutat Vella (Old City) which comprises the neighbourhoods of Barri Gòtic, El Raval, Sant Pere, Santa Caterina i La Ribera and La Barceloneta (http://bit.ly/BCNdistricts)
The RED route departs from the Columbus Statue and takes you through the Maremagnum Port Complex, the Barceloneta -the late 1700s neighbourhood created at the beachfront as a results of the citizens of La Ribera being displaced due the construction of La Ciutadella military citadel after the city fell to the Spanish king Philip V in 1714 (fortunately these days most of this citadel does no longer exist). Then a bit of the beach front and then towards the area of El Born in La Ribera neighbourhood, then Sant Pere i Santa Caterina and the Barri Gòtic neighbourhoods. Over 2000 years of history are packed within these three neighbourhoods and gather the most important buildings in the history of Catalonia some of which still are, from the remains of the Roman Temple of August to the medieval Palau Major where the Catalan kings have resided for centuries or the Palau de la Generalitat -roughly our equivalent to 10 Downing St or 1600 Penn Ave in DC- where the office of the president of Catalonia is located. The overall distance is 10.5km (6.5 miles) but this route is an extended one as is by far the most dense given the sheer number of things to see and to do, and all these alongside numerous shops, bars and other worthwhile visiting establishments, so plan for a good 6-8 hours at a relaxed path. Note this is a very dense area so don't be fool when planning your timings by the relatively short distances when looking at your map. This route includes, among many other: La Catedral de Santa Eulàlia, Santa Maria del Mar, La Ciutadella, Plaça Reial, Plaça Sant Felip Neri, Plaça Sant Jaume, Temple d'August, Plaça del Rei, La Rambla, Palau de la Música Catalana, Columbus statue, El Born, the beaches at La Barceloneta, Museu Marès, la Muralla Romana, etc...
The ORANGE route includes the neighbourhood of El Raval, once extra murs ('outside the walls' of the Old City), as well as a bit of Sant Antoni which technically is located in the lower corner of the Eixample district. El Raval had traditionally been the poorest area of the Ciutat Vella since the Middle Ages, where the outcasts and those not accepted by society (or wealthy enough to live in the city) used to live. This was, as I said, outside the walls of the city and when the city suffered attacks these lands were not under the protection of the city defenses. Oddly enough, this piece of land was an important one, as it was the 'garden of the city' and provided most of the fresh vegetables and fruits consumed by the citizens of Barcelona. One of the oldest hospitals in Europe was located here, the original Hospital de la Santa Creu, founded in 1401 by King Martí l'Humà and belonging to the Knights Templar Order. Today it hosts the Biblioteca (Library) de Catalunya as well as the prestigious Elisava School of Design. Both the interior patio of this complex of buildings and the library are while worth visiting. Today, El Raval is the most multicultural area of the city and for the past two decades has suffered a radical transformation that's slowly modernising the area. The lower part of El Raval, around Plaça Salvador Seguí, is where the Red Light District of the city is located. This route is 6km (3.7 miles) and contains several interesting landmarks such as the above mentioned Hospital de la Santa Creu, the ancient church of Sant Pau del Camp, Palau Güell, the Drassanes (royal shipyards), Palau de la Virreina, church of La Mercè, Sant Antoni market and La Boqueria market among other.