You're not doing what they did. You're doing something completely different. All they did was to change the colormap, not change the matrix or get a new matrix scaled to a different range. So all you have to do is to display your thermal image and apply a colormap and use caxis() to set the range to 30-40
Now a value of 30 in your image will be mapped to the lowest color in the colorbar, and a value of 40 will be mapped to the highest color. Values outside those limits 30-40 will take on the value of the color at the limit.