'teal' (the bird name) ultimately comes from Middle English 'tele' or 'teil(e)' used for a small freshwater duck; the color name 'teal' was later taken from the bird's characteristic blue-green (teal) markings.

'teal' appeared in Middle English as 'tele'/'teile' (referring to the duck) and the color sense developed in Modern English by naming the blue-green color after the duck's plumage.

Originally it referred specifically to the small duck; over time that distinctive blue-green head/marking of the duck gave rise to the color name 'teal'.