Manipulate[
 Show[ColorReplace[
   ColorReplace[Binarize[ThermalImage, {ImageMin, RangeMin}], Black],
   White -> Blue],
  ColorReplace[
   ColorReplace[Binarize[ThermalImage, {RangeMin, RangeMax}], Black],
   White -> Black],
  ColorReplace[
   ColorReplace[Binarize[ThermalImage, {RangeMax, ImageMax}], Black],
   White -> Red]], {{RangeMin, range1, "Range minimum"}, ImageMin,
  ImageMax, 0.1}, {{RangeMax, range2, "Range maximum"}, ImageMin,
  ImageMax, 0.1}]

imagemin = 0.3;
imagemax = 0.7;

Manipulate[
 cf = Piecewise[{
     {Blue, imagemin <= # < min},
     {Black, min <= # < med},
     {Red, med <= #}
   }] &;

 Colorize[thermimg, ColorFunction -> cf],

 {{min, 0.45}, imagemin, med, 0.01, Appearance -> "Open"},
 {{med, 0.67}, min, 1, 0.01, Appearance -> "Open"}
]