Precede active filament on status screen with letter F.

Firmware/ultralcd.cpp

@@ -533,7 +533,7 @@ void lcdui_print_extruder(void)
 {
 	int chars = 0;
 	if (mmu_extruder == tmp_extruder)
-		chars = lcd_printf_P(_N(" T%u"), mmu_extruder+1);
+		chars = lcd_printf_P(_N(" F%u"), mmu_extruder+1);
 	else
 		chars = lcd_printf_P(_N(" %u>%u"), mmu_extruder+1, tmp_extruder+1);
 	lcd_space(5 - chars);