I read the Apple's documentation and found that there is no public way to change the color of the checkmark.
So, in my according you can do it with an image. To do it, just set the accesoryView property of the cell to a UIImageView containing a checkmark of the correct color.
here is the code:
UIImageView *checkmark = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"checkMarkName.png"]];
cell.accessoryView = checkmark;