In both tkmedit and tksurfer, you set the min, mid, and slope threshold parameters to configure the functional/overlay color map. The min value specifies the values below which values will not be draw. So if the min is 1.0, values below 1.0 will be transparent, and 1.0 will be drawn in red. Values increasing from the min are drawn in colors approaching yellow. The value specified by mid is drawn in orange. The max value is drawn in yellow, and all values above this are drawn in yellow as well; the display is not clamped to the max. The max is determined by the slope:

        1.0
max = ------- + mid
       slope