You can only use the loops shown in green to create software instrument tracks. The teen loops are Midi loops. But GarageBand 10 does not offer many software instrument loops. This has been a lot better in the older GarageBand '11. It came with plenty of software instrument loops, which were just great. We could exactly see the chords that the loops playing (in the editor pane) and modify them according to our needs. And they need much less storage than the real instrument loops in of the current GarageBand version.
I got myself plenty of more instruments patches by purchasing MainStage 3. It installed the sounds and loops of the five Jam Packs. There are pianos in the Jam Pack World Music and Symphony Orchestra. But it has been more than a year ago, that I purchased MainStage, so I am not sure, if it still will install the Jam Packs, if you purchase it now.