The pair clashed at Barcelona prior to the midfielder's exit in 2010, and in the closing seasons of his eight-year career at the Etihad Stadium, Toure dropped down the rankings.
Now, in an explosive interview with France Football, the Ivory Coast international has accused Guardiola of being "jealous" of the footballer, and has also made allegations about the manager's treatment of African players.
"He was cruel with me," Toure said in the interview. "Do you really think he could've been like that with Andres Iniesta? It got to the point I asked myself if it was because of my colour. I'm not the first, other Barca players asked the question too.
"Maybe us Africans aren't always treated the same by certain people. When you see the problems [Guardiola] has often had with African players, everywhere he has been, I ask myself questions.
"He's too intelligent to be caught. He'll never admit [not liking African players]. But the day he picks a team with five Africans in it, I promise I will send him a cake!"
Guardiola took over at City in the summer of 2016.