You can use RGBAdjust inside ScriptClip, something like this (not tested)
Code:
ScriptClip("""
RGBAdjust(r=xyx_r, rg=xyx_rg)
""")
ConditionalReader("r.txt", "xyx_r", false)
ConditionalReader("rg.txt", "xyx_rg", false)
Edit - remember the variables are global, so give them names that are unique within the script (and any Imported scripts too)