GetPixel() not working correctly Windows API C++ - c++

Question

I'm writing a program that reads each pixel of a window and store it in an array of bytes as black and white, each bit of the bytes is a black/white value.
But GetPixel() doesn't seem to work the way I expected. Here's the part of the code for reading pixels and storing them:
byte *colors = new byte[250000 / 8 + 1];
ZeroMemory(colors, 250000 / 8 + 1);
HDC hdc = GetDC(hwnd);
HDC memDC = CreateCompatibleDC(hdc);
HBITMAP memBitmap = CreateCompatibleBitmap(hdc, 500, 500);
SelectObject(memDC, memBitmap);
BitBlt(memDC, 0, 0, 500, 500, hdc, 0, 0, SRCCOPY);
for (int y = 0; y < 500; y++) {
for (int x = 0; x < 500; x++) {
COLORREF pxcolor = GetPixel(memDC, x, y);
if (pxcolor == CLR_INVALID) {
MessageBox(hwnd, _T("Oops..."), NULL, NULL);
}
int r = GetRValue(pxcolor);
int g = GetGValue(pxcolor);
int b = GetBValue(pxcolor);
int average = (r + g + b) / 3;
bool colorBW = average >= 128;
int currentIndex = y * 500 + x;
if (colorBW) {
SetBit(colors, currentIndex);
}
}
}
ReleaseDC(hwnd, hdc);
DeleteDC(memDC);
DeleteObject(memBitmap);
delete[] colors;
SetBit():
inline VOID SetBit(byte *bytes, int index, bool state = true) {
byte byteToSet = bytes[index / 8];
int bitNumber = index % 8;
bytes[index / 8] = state ? (byteToSet | (0b1000'0000 >> bitNumber)) : (byteToSet & ((0b1111'1111 >> (bitNumber + 1)) | (0b1111'1111 << (8 - bitNumber - 1))));
}
Every pixel read in by GetPixel() seems to give me 0x000000, or pure black.
My code used to call GetPixel() with the first parameter being hdc, without all the bitmap and memory DC stuff, but that way every pixel returns CLR_INVALID. I came across this question, and the above code is after I have changed it into using memory DCs and bitmaps. But it just went from returning CLR_INVALID to 0x000000 for each pixel.
If I add this line before I use GetPixel():
SetPixel(memDC, x, y, RGB(255, 255, 255));
GetPixel() returns the correct result. Why is it functioning this way?