I have 2 ideas:
1) create a shared string that is delimited and holds both car and sum...I've used this before and for me it would look like
|BMW@3|Audi@1|
then it is all string parsing and manipulating...new values are just added to the end

the other idea is to have 2 arrays, one with the name of the car in it and the other with sum

or a melding of the 2 solutions an array with a delimited string holding the name and the sum.

you could search the array fairly easily with something like:
if left(array[x], len({table.carname}) = {table.carname} then do something.

both of the last 2 suggestions with arrays would of course be done in a for loop with variables for found etc so that new cars are added to the end.

HTH

wouldn't that give the same value all the time?

which if that is what is desired...
it just looked it was a running total grouped on color then car.

i've been wrong before, so it wouldn't surprise me