Question

    RECT rect;
    HWND hwnd = GetDesktopWindow();
    LockWindowUpdate(hwnd);
    HDC hdc = GetDC(hwnd);
    HBRUSH hbrush = CreateSolidBrush(RGB(255,0,0));
    GetWindowRect(hwnd, &rect);
    FillRect(hdc, &rect, hbrush);         

This is my previous attempt which did nothing. Just to be clear, I am trying to color the whole screen a solid color.

Answer 1

This code works using GDI (which you seem to be using):

#include <windows.h>

int main() {
    HDC screenDC = GetDC(NULL); //NULL gets whole screen
    HBRUSH brush = CreateSolidBrush(RGB(255,0,0)); //create brush
    SelectObject(screenDC, brush); //select brush into DC
    Rectangle(screenDC, 0, 0, GetSystemMetrics(SM_CXSCREEN), GetSystemMetrics(SM_CYSCREEN)); //draw rectangle over whole screen
    //clean up stuff here
    return 0;
}

This is meant as an example, not a robust solution. For example, selecting the previous brush back into the DC is always a good idea. So is releasing the DC. Another option is to have a fullscreen window sitting over top of everything that you can use to draw onto.