As per general rules of naming plants genus and species epithets should rhyme. So, ideally when Panicum colonum was transferred to new genus Echinochloa, the right naming should have been Echinochloa colona. However, certain authors have preferred to conserve the original specific epithet colonum. So, I have seen many authors use Echinochloa colona while many others use Echinochloa colonum.