You could create a Bitmap, create a Graphics object from it and fill it with any color.
Bitmap^ bmp = gcnew Bitmap(WIDTH, HEIGHT);
Graphics^ g = Graphics::FromImage(bmp);
g->FillRectangle(
gcnew SolidBrush(Color::FromArgb(alfa, red, green, blue)),
Rectangle(Point.Empty, bmp.Size() );
So easy.