NOTE : This is just safe code and will be slow. I will change it to unsafe later but the following code is just for showing the idea ;).
void ChangeColor(Bitmap image)
{
for (int x = 0; x < image.Width; x++)
{
for (int y = 0; y < image.Height; y++)
{
Color c = image.GetPixel(x, y);
Color nc = Color.FromArgb(c.A, c.R, 0, 0);
image.SetPixel(x, y, nc);
}
}
}