The left eye looks similar to the right eye in terms of appearance, but the left eye has a pink ponytail instead of a tuft of hair. Her iris isn't black like the right eye's, it flashes through all the colours of the rainbow, this apparently letting her and the right eye be able to see colour.
However, her purpose for letting the right eye be able to see colour is defeated by the Awesume Glasses.
The left eye was captured by the pirate cloud and taken away from her boyfriend, the right eye. The right eye traveled through the Vitreous Woods, Iris City, and the Pirate Factory to try to save his girlfriend, who was constantly being carried away by the pirate cloud.
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Eventually, the right eye reached the pirate cloud. The pirate cloud then placed the left eye in his right eye socket and fought the right eye. The right eye then defeated the pirate cloud, releasing the left eye.
The left eye does not actually affect the gameplay of Colourblind at all. She is only seen at the end of levels one to nineteen, being carried away by the pirate cloud to the next level.
The left eye with pirate cloud
Left eye in the intro
Left eye being kidnapped by the Pirate Cloud.
The left eye as seen in the introduction of Colourblind