Select two images of the same size and adjust the diff error ceiling to be greater than or equal that the current max error. If you want to have a non-linear scale, e.g. to highlight the sections of low difference, adjust the remapping exponent.
If the error is larger than the defined error ceiling a green pixel (■) is shown instead, so you know that the error ceiling is set too low. Why doesn't this app just adjust the ceiling for you? Well, if you want to compare many images with the same gradient applied to all of them you want to be able to fix it ceiling manually.
The diff for a pair of RGB pixels, X' and Y', is defined as the absolute error in sRGB relative luminance* between the two, as follows:
X = sRGB_gamma_decode(X')
Y = sRGB_gamma_decode(Y')
LX = sRGB_luminance(X)
LY = sRGB_luminance(Y)
error = abs(LX - LY)
remapped_error = error / error_ceiling
diff = pow(remapped_error, exponent)
* Note that this implies that both input images are in the sRGB gamma-encoded color space. If you are unsure what that means, it's probably okay to ignore it.