Don’t use a random.
Feed it 3x Per Instance Custom Data, push the floats through a Vertex Interpolator and make a float3.
edit: it’s actually in the video you linked.
edit2: here:
Use the desired RGB rather than a random.
Is this not working for you? Also, the dirty flag should be set for the last instance only. Using it like this almost defeats the purpose. And that should be Instance Count - 1.