Dodge offers Brown and tan because Clinton County Sheriff's Dept has them (unless they are custom)
Dodge will supposedly do custom paint at the factory for quantity orders. Not sure what the minimum is. I talked to one of the Johnson County sergeants a while back not long after they got their first marked Chargers, and he said they had to have them custom painted because they couldn't get the traditional sheriff two-tone done at the factory. Maybe with quantity orders (several counties going together, perhaps) it is happening now.
Marion County Sheriff's Department has a good number of Chargers in service (not IMPD, MCSD, yes, they still exist) both marked and unmarked. Their marked Chargers are a bronze color with a simple traditional sheriff's star door shield, and a Sheriff decal on the trunk lid.
As far as any paint scheme being mandated by state law, there's no such animal. It may have been in the not so recent past, but not in the past ten years or so. The sheriff two-tone is, to the best of my knowledge, an adopted standard of the Indiana Sheriff's Association. There have been variances county by county through the years, but more so recently because the two-tone costs so much. The only state law dealing with two-tone paint schemes is
IC 5-22-22-9, regulating disposal of law enforcement vehicles. It requires that a vehicle model year 1994 or newer with a two-tone paint scheme similar to that used by a law enforcement agency in Indiana be painted a solid color if it is to be registered and/or operated by a private party on a public highway in Indiana. This could mean brown and tan, black and white, or pink and purple if there was some agency somewhere in the state that used it.