I only know that it uses RGB info, I guess that counts as colour. But not in the way we use our eyes (or rather the brain) to tell if a picture shows the same. In our case, the difference in resolution can be enormous because we just fill in the gaps in our mind, But RC needs to identify certain features. I am pretty certain that those features are constitutet by a distinct distribution of colour changes in a certain area. This means that the same change needs to be identifiable in two different images, in our case with changing resolution. And then it makes sense that the steps shouldn’t be too big.