Hi there. Not sure this will help, and I'm only speaking from my experience.
From wikipedia on
Spectogram, we have, a spectogram common format "..is a graph with two geometric dimensions: one axis represents
time, and the other axis represents
frequency; a third dimension indicating the
amplitude of a particular frequency at a particular time is represented by the
intensity or color of each point in the image."
So yellow/red colors are higher volume than green/blue/purple.
In terms of quality, and assuming the question has to do with an original highest quality source encoded to lower qualities, I look for the file with the highest frequency.
In the example you provide, you can see that the mp3 peaks at a lower frequency than the flac. Also, if you observe the peak frequencies of the mp3, you will see the somewhat loud green frequencies abruptly terminated in the mp3. There is more of a fade to higher frequencies in the flac file. And finally, the more intense colors in the low end could be the result of the known muddy effect introduced by lame encoding.
In cases where the files are not from equal sources, you may see greater loudness due to DSP effects like wavhammer that bring more quiet sounds from the background into the foreground, and it is a matter of taste at that point whether the louder potentially lower peak frequency file is better quality or not.
In short, loudness does not mean more quality, except when it does.
R