Training a Special Ability or New Position

[Toxcatl]

about underlined part, is that tested? is it old version feature that doesn't work in same way now?

There is a guide that explains it: https://forum.topeleven.com/tutorial...ng-system.html
In short:

XP from a drill goes to all its skills equally, if they are all key skills. For example, Hurdle Jumps is a Hard drill (4 XP) and trains 3 skills (Bravery, Aggression and Speed) for 4 / 3 = 1.33 XP each on level 1, if all these skills are white.
If there are grey skills, they receive 25% less experience from training, and this XP goes as bonus to the white skills of this drill equally. So a drill with grey skills trains its white skills faster, but for the cost of XP spent on grey skills. Total quality gain of the player is unaffected: drills of the same level improve player's quality with the same rate regardless of having or not having grey skills.

why I ask is I have a player in his 3rd season now. so far invested 240 greens in him to train drill which gave him 3 whites with 1 gray skill, crossing. I'm comparison to my other players of same Q dont think he got less crossing then players which have it as white skill.

All players get XP with different pace. You have to use a single drill on a single player many times (or GKT + drill), write down his gain in each skill, and then compare how much he got. Then you'll see.

other part of question would be how sustainable is to add new position just in order to highlite desired gray skill.

I suspect that it doesn't work like this. I think if you have DC and give him DMC to highlight Passing, but you keep using him on DC position, that Passing will be as good as grey. Even worse -- it will consume more XP.
White skills are not skills a player uses, they are skills that are useful on the position(s) he's playing. If he doesn't play on a position with Passing, his Passing is either useless or less important regardless of being grey or white. But this is not a science, you know, just a speculation. You may be right here, I'm just pointing out we don't know how does it work, and there is a chance it doesn't work like you assume.

also, if he remains to play as DC, will he use that skill any more than before?

Do players use their skills at all? There is no evidence of that. Maybe the average is considered, maybe the sum, maybe only quality matters.