The material operator allow you to choose which Material ID you wish to replace, so in the case of the 04 f preset we need to enter a value of 6 as this is the ID used for the seat colour.
You can then either change the IDs on the splines as in the previous example, so that they use the correct Material IDs from the Multi-Sub object material. Alternatively you could use IF statements to map the spline's IDs to a different number. An example of an expression that map Spline IDs 1 - 3, to material IDs 6-8 would look like this:
I've attached this example below. I hope that helps, please let me know if you need anything else.