if there is any red they could be crossed
brown is actually a recessive gene and is easily bred out if they are too close to a red bhut, also sometimes they will revert back to red as there is always a red gene in the mix, brown and red are colors very close to one another
green to brown it the real choco bhut i had this same problem with one of my choco bhut the first one i got turned green then a little brown and then fully red
hope this helps
thanks your friend joe