How I use them:
Red arrows - when player, usually winger, has open space in front of him (eg. when I have 4 wingers while opponent has only 2)
or when I choose defensive formation to force players to join attack.
Blue arrows - when I'm focused on defense or outnumbered in some part of pitch (eg. when I have 2MC+DMC in the middle while opponent has AMC+2MC+DMC, then I use blue arrows on MC's)