var filter = new GrayscaleToRGB();
var rgb = filter.Apply(img);

byte[] grabResult = IGrabResult.PixelData as byte[];

byte[] RGB = new byte[1920 * 1200 * 3];
int StartIndex = 0;
// für Grün
for (int Index = 1; Index < RGB.Length; Index += 3)
{
RGB[Index] = grabResult[StartIndex];
StartIndex++;
}
// für Blau
for (int Index = 2; Index < RGB.Length; Index += 3)
{
RGB[Index] = grabResult[StartIndex];
StartIndex++;
}

  [StructLayout(LayoutKind.Sequential, Pack = 1)]
    public struct RGB24
    {
        public byte B;
        public byte G;
        public byte R;

        public RGB24(byte value)
        {
            B = value;
            G = value;
            R = value;
        }
    }

    public class ImageConverter
    {
        private RGB24[] _lut;

        public ImageConverter()
        {
            _lut = new RGB24[256];
            for (int i = 0; i < _lut.Length; i++)
            {
                _lut[i] = new RGB24((byte)i);
            }
        }

        public unsafe byte[] ToRGB24(byte[] data)
        {
            byte[] result = new byte[data.Length * 3];
            fixed (byte* bptr = result)
            {
                RGB24* ptr = (RGB24*)bptr;
                for (int i = 0; i < data.Length; i++)
                {
                    ptr[i] = _lut[data[i]];
                }
            }
            return result;
        }
    }