"So, what conclusion can we draw from this? Will a player gain less if he participates in a drill that train abilities that are grey for him?
I have actually found that this is not the case, at least contrary to my believes before doing these tests. The grey abilities for my player Dailly -- Crossing and Finishing -- are covered only by the Fast Counter Attack drill in this practice. As the table show, the decrease in gain in Crossing and Finishing have instead almost to the decimal been attributed the other abilities covered by the Fast Counter Attack drill, ie Passing and Creativity.
This can be explained with the following example. Lets say we run a large number of Fast Counter Attack drills (this drill improves Creativity, Crossing, Finishing and Passing) on a AMR (Creativity, Crossing, Finishing and Passing are all highlighted on a AMR), we could get the following result (plus minus only a few percent):
Creativity +50%
Crossing +50%
Finishing +50%
Passing +50%
But if we instead runs the same amount of drills on a MC (Only Creativity and Passing are highlighted while Crossing and Finnsihing are grey). we should roughly get the following result:
Creativity +65%
Crossing +35%
Finishing +35%
Passing +65%
2.2 Mix of drills
I have found that the mix of drills does -- not -- impact the Training Efficiency Factor at all. As is well known, you get a higher bonus (Offense, Defense, Conditioning and Possession) from a well constructed practice. But the individual gain in ability is not impacted by the mix of drills in a session.