Once the essential najasat is removed from the skin, then other objects will only become najis if the skin transfers smell color or taste of the essential najasat right?

It’s very clear if the essential Najasat has been removed but it is still has smell, it means it has not been removed totally because having smell means small particals are still there even we can’t se them by our naked eyes. But yes if color strains has not removed fully specially in case of blood when it spreads on clothes and we have washed it but still strains of blood are there it would be considered Pak ( Tahir), so smell and taste is different from the color.
I hope you have understood inshallah. For example if one has faeces spread out from his anal area and he has cleaned it with tissue paper, it can’t be considered Pak unless he cleans himself with water but if the original najasat was around the anus area it can be cleaned with tissue paper. Same rule we apply on other parts of body but sometime Najasat has very strong smell even if you was it with water still smell unless one has washed it with soap etc then it’s ok that smell will not be considered that the surface is still Najis.