Possible to pipe string to an external command and show colored output?

Basically I want `:h :write_c` , but put a string from a variable into stdin, not some lines in a file.

I'm trying to color listed text using Generic Coloriser - grcat.

This outputs the color codes literally as `^[[0m`

echo system("grcat ~/.grc/conf.vimbuflist", buflist#lastused#get()->buflist#format_buffer_names())