Topiramate
Topiramate is the drug which might be responsible for this malformation.
Topiramate is an anticonvulsant drug which is taken alone or with other drugs to prevent and control epilepsy. Topiramate is produced in four forms and are all taken by mouth. Topiramate has many side effects which include birth defects if is taken during pregnancy, osteoporosis, vision loss, nervousness and upper respiratory tract infection.
We need to know the dominant and recessive traits of the noombats in order to answer this question. If both of them has yellow as the dominant trait, then green may be recessive. In that case there is a chance for them to have a green bellied child. Its impossible to tell the percentage though because we don't know enough about the animals.
Assuming purple is dominant, the genotypes of the parents must be Pp and pp, such that half the progeny have the dominant allele (P_) and half have the recessive phenotype (pp) 25% possibility of white-flowered plant life and 75% possibility of pink ones
