Init old_red to 127 to ensure initial change

Fix #10012

Marlin/src/gcode/temperature/M140_M190.cpp

 
   #if ENABLED(PRINTER_EVENT_LEDS)
     const float start_temp = thermalManager.degBed();
-    uint8_t old_red = 255;
+    uint8_t old_red = 127;
   #endif
 
   do {