@chicken vector gives good advice: the handle h returned by a graphics command generally has useful hints about what you can customize about how it is displayed. Sometimes it's still not obvious what all of the available options are, though. In that case there is often more detailed documentation that you can access by
doc h
If you run that after the example you pointed to, and go down to the section about CellLabelColor, you will see
> Text color for data labels, specified as 'auto', an RGB triplet, a hexadecimal color code, a color name, or a short name. The default value of 'auto' chooses an appropriate text color, depending on the color of each heatmap cell. If you do not want the labels to display, specify 'none'.
So in other words, when this is set to auto (which is the default), it will choose a light color text for cells whose backgrounds are dark, and vice versa. It looks like there is no documented/straightforward way to choose particular colors for particular ranges of values.
