fast trainers give the added value in your team. Use red boosters to your fast trainers to fix the injuries asap.
Don't give greens to slower trainers, only if you want em to be ready before a match. Give them training if they are full and you have time before a match.
Don't train the players after a match. Wait at least 1 rest period (3h). Green boosters is "fake". The players are still tired.