With the following code, the background color is not applied:
!define TestColor red
skinparam classBackgroundColor PaleGreen\TestColor
All other variants that I tested are working:
skinparam classBackgroundColor PaleGreen/TestColor
skinparam classBackgroundColor PaleGreen-TestColor
skinparam classBackgroundColor PaleGreen\red
skinparam classBackgroundColor PaleGreen-#F00