There are four metro lines in Budapest. The yellow is the oldest one, which in fact is the second oldest subway/metro line in Europe. The red is the third and the green line was opened on March 28th in 2014. On the map on this page you can see all the four metro lines, and you can also see the lines of some of the city trains (HEV lines) in Budapest.
If you click on the maps you can see them in even bigger version. Enjoy your travel in Budapest using public transportation and the metro..