The data variable uses colour progression to represent itself in each region of the map. Typically, this can be a blending from one colour to another, a single hue progression, transparent to opaque, light to dark or an entire colour spectrum.
One downside to the use of colour is that you can’t accurately read or compare values from the map. Another issue is that larger regions appear more emphasised then smaller ones, so the viewer’s perception of the shaded values are affected.
A common error when producing Choropleth Maps is to encode raw data values (such as population) rather than using normalized values (calculating population per square kilometre for example) to produce a density map.