How does spectrogram color axis work?

I'm not sure how the color axis works when taking the spectrogram of a signal. I'm sure that the color scale is a dB scale but what is the reference signal? Perhaps the colors are such that the most intense frequency component is given the most intense red corresponding to 0dB and then all other components are assigned a color relative to this reference?

In any case I don't know how to proceed with the following problem. I have a reference signal, call it X and another signal call it Y. X is a constant amplitude chirp signal so on the spectrogram of frequency vs. time it is just a straight line of constant colour, frequency increasing linearly with time. Y, on the other hand is similar to X but the frequency components are not constant in amplitude (it is mostly an attenuated signal).

Now what I want to do is to only plot my Y signal, but to have the colour scale such that X is the reference signal, in other words 0 dB (which could maybe correspond to a color in the middle of the ROYGBIV scpectrum) on my colorscale for the Y signal should mean no attenuation/gain. Would be extremely grateful for some input on how to do this! Thanks.