You need to analyze the original grayscale image, not the rgb pseudocolored image. To verify you should look at pixel values and compare the intensity at that location with the temp measured with another device, like a thermometer, and use on individuals known to have, or not have, fever.
To apply pseudocolors, use the colormap() function. There are a variety of built-in colormaps you can choose from, in addition to building your own custom colormap.
