You'll get one main effect for each variable in your experiment. In addition to main effects, you'll also be able to look at interactions. In the absence of a formal definition, interactions have to do with the combined effects of variables. You'll get one interaction for every combination of variables in your experiment. In our case, there's just one, the mood X room color interaction.
Nothing operates alone.
Consider your performance on the last exam. Study-time certainly influenced your score, but so did the amount of sleep you got the night before, whether or not you had breakfast, how crowded you were, etc. All of these separate causes could be investigated in an experiment, factorial designs are built around that idea. Instead of doing one little experiment for each variable we can do the whole thing in one big experiment. This can result in great savings.