The Newcastle United midfielder was told last summer that he was not in Rafa Benitez's plans.
However, the 28-year-old wasn't able to secure a move away from the club in the last transfer window.
Colback, signed as a free agent after his Sunderland contract expired in 2014, has trained at the club's Academy for the past five months.
Most Popular
-
1
The 16 places where most crime was reported across South Tyneside in June
-
2
‘Bully’ broke disabled girlfriend’s arm and dragged her through broken glass in shocking campaign of cruelty
-
3
‘Sinister’ paedophile watched children play and offered them doughnuts after moving into new neighbourhood
-
4
Extension plans rejected for South Shields home
-
5
13 great pictures as the sunshine sees crowds flock to South Shields beaches
The Killingworth-born player has made a handful of appearances for United's Under-23 side.
Forest are understood to have agreed a loan deal with Newcastle for Colback, who has another year left on his St James's Park contract.