Why not "una tipa"? Logic being "rosa" as a flower is a feminine noun when it means a type of flower and when it means the color pink it is a masculine noun. Here it is a flower. "un tipo de rosa" has 47,000 results google results and "una tipa de rosa" has a mere 29 so I do assume duolingo is right. Ah never mind I see "tipo" is masculine when it means "type".