The colormap is a mapping of intensities to a color scale, and the +/-1 is just a multiplier of the range value. The range sets the maximum to apply to both positive and negative values. Autorange sets the range to the largest absolute value in the overlay sub-brick. Setting "Pos only" will limit values from 0 to the maximum range only for non-negative values. You can override the multiplier value if you like (right-click over top of overlay colorbar, 'set top value'), but that's rarely useful; it's usually more straightforward to set the range.