I imported ede-scriptbus on the repo (it is in branches), adding few new stuff: setting XSETTINGS values and explicit XSETTINGS serialization, which was done implicitly when evoke quits. For example, to change current background color of EDE apps, this will be used:

./ede-scriptbus

> (set-xsettings "Fltk/Background" '(R G B A))

where, as you guessed correctly, you are using separated color components. The good thing about (set-xsettings) is how it will figure out parameter type and send correct message to evoke.

Due the nature of XSETTINGS protocol, changes will be instantly applied.

Too bad Gtk+ (as it implements XSETTINGS) does not implement some keys for changing foreground and background colors, so we could apply color changes on Gtk+ apps too.