Today, Lauren Anderson (Flatiron) showed me some awesome visualizations of the color-magnitude distribution (CMD) of stars as a function of position in the Galaxy. There are variations! The question is: What part of the variation is due to dust and what part is due to metallicity (or composition) and what is due to star-formation history (or age)? We don't know how to answer these questions! But one thing we did is take some derivatives of the CMD with respect to position. Is this a sensible thing to do? Could we treat the CMD as data and try to fit it with a latent-variable model? That is, what is the right approach to quantifying and interpreting the CMD variations around the Galaxy?