Coloring speccific values in a spectrogram

[countersubject]

Hello,
I have the following situation: I have a spectrogram for which I use a color map with color stops. It works fine, but I want to use the color grey for 0 values returned by the value function that needs to be overloaded. The interval in the plot widget may or may not include the zero, but I want all pixels in the spectrogram, where 0 was returned, to be grey. Color stops work only in the interval range, but even if they didn't I need only one value to have a color, not a range of values(even a narrow one).
How can I accomplish this?
(attached is a screenshot that shows what I want to do)

Thank you