What is seismic tomography?
Much like how a CT scan can image the inside of a human body using electromagnetic waves, we can use elastic waves from distant earthquakes to image the inside of the planet. This allows us to differentiate areas of high and low seismic wave speed, which we in turn interpret as various geologic constraints (rock type, temperature, partial melt, viscosity, etc.).
When you see tomographic images, typically they show slower seismic wave speed regions as red and faster seismic wave speed as blue. This is because seismic velocity for similar materials is most dependent on temperature over other physical characteristics. So you can think of the blue areas as colder and the red areas as warmer.
Hopefully this will help you more easily understand what some of the figures I’ve posted here represent. Enjoy!