
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.
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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.