Suppress ColorPrint for the multi-material printing

src/libslic3r/GCode.cpp

@@ -1326,7 +1326,7 @@ void GCode::process_layer(
         m_colorprint_heights.erase(m_colorprint_heights.begin());
         colorprint_change = true;
     }
-    if (colorprint_change)
+    if (colorprint_change && print.extruders().size()==1)
         gcode += "M600\n";