Its something to do with balance. So You are given a bunch of stuff to exercise on and your stats are split 3 ways. Each bit of equipment does something positive or negative to the 3 stats. Generally Red means you ave had enough and time to switch equipment until you get too tired its just a case of finding what fighter style you want and train the stats accordingly, but bear in mind its a slog/grindy as hell and the whole thing of balance is evident throughout your play-though Food Money training sleep etc

Little bit late but in case anyone needs to hear it, sad faces means your happiness is low so training is less effective. Go hang out with Roy or something