Fill background with color (black) for cheap?

hi everyone . i need to fill with black color entire display to simulate that device is dead. so i can use image with only black color in it but maybe there is another trick to temporarly show black screen on top layer?
8+ version

What MCU/Processor/Board and compiler are you using? You can turn off the screen if you want.

custom stm32h7 with 8080 lcd 320x240, c compiler.
yes i have pwm backlight and so on. but its not an option for me to just turn off lcd. now i am using black wallpaper.

actually i dont have time to play with lcd and its registers so i thought there are some easy and cheap solutions to fill lcd with color.

Try this:
lv_obj_t * black_screen = lv_obj_create(NULL, NULL);
lv_obj_set_style_local_bg_color(lv_scr_act(), LV_OBJ_PART_MAIN, LV_STATE_DEFAULT, lv_color_hex(0x000000));