I do not only train grey, but Drills that involve 1 white skill and 2 grey skills for example, like in the example above Hurdle Jumps for Speed, Bravery, Aggression.
It is, because in my opinion, overall quality % is still the Main factor when 2 Teams meet, not only skills. I like my players to have white skills all around 200% but not much above as it is expensive to train and for me not so effective. So I prefer to train some grey later in the players career for cheap to maintain a high overall %. It is my philosophy, other people will tell and do Different.
Thats why I also State it is good to train players 22-25 yo if you have a rather unbalanced Player in terms of skills and so are able to find good Drills, that are on a lower average.