Thank you very much! It worked. Now, for anyone else interested here`s how the symbols are numbered:

0_1 4_5
2_3
So it`s just a matter of changing the index number of each culture from descr_culture to the one you want
Code:
culture barbarian
portrait_mapping barbarian
rebel_standard_index 1
And the culture of the settlement is indeed that of the faction mentioned in descr_regions for the culture line of each region/settlement:
Code:
Maeotis
Tanais
scythia
Maeotae
131 89 33
grain, slaves, dogs, pigs
5
9