It's hard to distinguish the two species just by looking at their colour.

distinguish 是动词，意思是"区分、辨别"，由前缀 dis-（表示"分开"）和词根 stinguere（拉丁语，意为"刺、戳"）组成，字面意思是"通过标记分开"，引申为"辨别、区分"。