The stats that are in white should be trained and the grey stats can be ignored.
It's not that the grey stats are usless.
For a ST fitness is grey but I train it as fitness is important for any player. Fo attacking midfielders strength is grey but I train it because many times attacking midfielders are in one on one situation with the defenders.
But stats like Crossing for a ST, AMC, MC, DC are completely usless.