Two people walk up to a brick building with a colorful mural on its side.
Two people walking toward a brick building with a colorful mural outside.
Sentence two gives more detail in the fact that the brick building has a mural on the outside. A more specific version of side
The mural would be on the side of the building outside since people were walking up to the building it was on.
People working in a building with a colorful mural.