Most spectrograms are displayed in shades of gray. The darker the shade, the more energy at the corresponding point in the time-frequency plane. Figure 1 below is a gray-scale spectrogram for the word "compute."
Personally, I think that color spectrograms are much easier to decipher than gray-scale spectrograms. I'd be glad to hear any dissenting opinions on this point. Don't expect me to give in easily, however; I have a lot invested in my color spectrograms.
Figure 1 - A gray-scale rendition of the spectrogram for the word "compute."