SDL SetRenderDrawColor Opacity doesn't work - c

Question

I have an issue with my code. I don't manage to change the opacity of my RenderDrawColor.
Indeed, I know that SetRenderDrawColor is defined by (renderer, R, G, B, a --> which represents the opacity value). Nevertheless, I tried to change a value but my color doesn't change and I have my rectangle fully fill again.
Here a photo of what I have with my code :
Screen that I have with my code
Here is my code, that I execute with a main.c script :
void display_end_menu(SDL_Renderer *renderer) {
if (SDL_SetRenderDrawColor(renderer, 0, 255, 0, 30) != 0) {
return_error("Impossible de modifier la couleur du render");
SDL_Quit();
}
SDL_Rect shadow_screen;
shadow_screen.w = 1280;
shadow_screen.h = 720;
shadow_screen.x = (SCREEN_WIDTH - shadow_screen.w)/2;
shadow_screen.y = (SCREEN_HEIGHT - shadow_screen.h)/2;
if (SDL_RenderFillRect(renderer, &shadow_screen) != 0) {
return_error("Impossible de dessiner le rectangle dans le render");
SDL_Quit();
}
if (SDL_SetRenderDrawColor(renderer, 255, 255, 255, 30) != 0) {
return_error("Impossible de modifier la couleur du render");
SDL_Quit();
}
SDL_Rect quit_button;
quit_button.w = 200;
quit_button.h = 50;
quit_button.x = (SCREEN_WIDTH - quit_button.w)/2;
quit_button.y = (SCREEN_HEIGHT - quit_button.h)/2;
if (SDL_RenderFillRect(renderer, &quit_button) != 0) {
return_error("Impossible de dessiner le rectangle dans le render");
SDL_Quit();
}
}
I tried several numbers for alpha such as 0, 30 and so on, but it's impossible to edit the opacity of the rectangle.