go to dsecr_sm_factions.txt and find venice.
It should look like this:
Change it to have the papacy colors:
This was to make the map colors the same as the Pope. For units, you are going to have to go to the battle_models.modeldb and make all venice units use the Papal Textures
For example, this is the original code for Venice's Italian Spear Militia (red indicates parts of interest, blue indicates change):
and this
You need to change them both to where they look like this
and this
This will make your Italian Spear Militia have the same colors as the Pope.
Hopefully this helped you.