By default, the TreeMap shows value intensity as a gradient of a single color.
This Monochromatic Gradient can be set to a different color by using the dedicated combobox under Range Color Settings :
The TreeMap can be configured to show value intensity in ranges of single colors.
Select Range Brush Items and choose the colors of each interval :
The interval between the maximum of Value and the minimum of Value will be split in equal intervals and assigned a single color.