The colour pallet is written into the code and is not user selectable other than the standard colours / greyscale option.
Black represents the most intensity and lighter shades of grey indicate lesser intensity.
The “Gain (dB)” setting in Spectrogram preferences determines the signal level at which black occurs. When “Gain” is set to zero, a 0 dB sine wave will appear as black. Increasing the Gain setting provides a “sensitivity” adjustment, so, for example, if the maximum level of any frequency is -20 dB, and you want that to show as black, you would need to set the gain to 20 dB (+ 20). What the Gain setting does is to amplify the signal by the specified amount prior to analyzing. This allows you to set the high (maximum) end of the scale.
The low end of the colour/greyscale spectrum is set by the “Range (dB)” setting. For example, if that is set to 50 dB, then the lightest shade of grey will appear at 50 dB below the maximum (as set by the “Gain (dB)”)