I'm afraid there's no direct correspondence between the 'seed' value and the ID assigned in the Forest Color map. The seed is a large integer used to initialize the random generator, which primarily affects transformations.
For your scenario, you can use the Forest Color map for most cars and, where specific control is needed, duplicate the item in the Geometry list, assign the desired material, and switch the item accordingly. Optionally, you might find the 'Match Color ID on Map' Diversity mode helpful - it could suit your needs.
I hope that helps.
Best regard,