[quote=@Joshua Tamashii]
You sir, have some required reading: [url]http://fairytail.wikia.com/wiki/Ice_Devil_Slayer_Magic[/url]

Second paragraph, second sentence.
[/quote]

"Able to somewhat control the ice of others" seems to me like it's questionable. I'm re-reading the chapter that was referenced, but I'm inclined to believe that it's only because Gray's ice magic is a normal type, not a Slayer type. 

Phantom Lord's fire mage, Totomaru, had an ability to control other types of flames too, and was able to use it on Natsu's Dragon Slayer flame, but afterwards Natsu wrested control back. No doubt a Devil Slayer equivalent of that ability would be stronger, but Slayer elements have some sort of different quality to them. If Gray had been an Ice Slayer of some sort I'm betting Silver wouldn't be able to take his ice from him without a special method like Natsu's special method of eating God flames.

Edit: And after re-reading the chapter that was referenced, I think this is entirely incorrect on the Wiki's part. Nothing in that chapter showed Silver taking control of Gray's ice. He ate some of Gray's attacks and then spat more ice back at him, the same way Natsu has done to other flame mages--but only with a special method to another Slayer. The only instance where he might have "controlled" one of Gray's attacks is when Gray tried to channel his own Slayer ice back at him, and Silver was unaffected. This doesn't point to him controlling Gray's attack, but his own magic. If that's the case, then reversing War's own magic will indeed not work, but it doesn't mean War can seize control of anyone else's magic of the same/similar element.