In this expression node I can’t use functions in code, but they are very necessary, especially for reusable code in shader…
Also, I can’t use external functions, which are declared in the graphic editor of materials.
For this reason, the only way out - use of plugins or some dirty hacks like this code in Custom Expression Node:
#include "/Engine/Private/iacopoHlslStructures.ush"
return iacopoHlslFunctions.OrangeBright();