OSC commands returning the colour sequence must be terminated by either
of ST, ESC \, or BEL. rxvt-unicode tries to use the same type of termination
as was in the query but doesn't correctly handle the multiple-char sequence
used for 7-bit queries. Force to using ESC \ instead for now.

Index: src/command.C
--- src/command.C.orig
+++ src/command.C
@@ -3426,9 +3426,9 @@ rxvt_term::process_color_seq (int report, int color, c
         snprintf (rgba_str, sizeof (rgba_str), "rgb:%04x/%04x/%04x", c.r, c.g, c.b);
 
       if (IN_RANGE_INC (color, minCOLOR, maxTermCOLOR))
-        tt_printf ("\033]%d;%d;%s%c", report, color - minCOLOR, rgba_str, resp);
+        tt_printf ("\033]%d;%d;%s\033\\", report, color - minCOLOR, rgba_str);
       else
-        tt_printf ("\033]%d;%s%c", report, rgba_str, resp);
+        tt_printf ("\033]%d;%s\033\\", report, rgba_str, resp);
     }
   else
     set_window_color (color, str);