There was a short story I read years ago that has stayed with me in the back of my mind; I just can not think of the title. It is about an old woman who by happenstance meets a group of humanoid aliens. She is very kind to them, and in doing so they decide to be hostile and aggressive to humans. The twist was at the end of the story the reader realizes that because the old woman was colorblind, she could not see the alien's inhuman skin color.
This is Minister Without Portfolio by Mildred Clingerman. I no longer have the book in which I read this story, but this is the abstract I wrote after I read it:
A woman quite by chance meets these nice men in the forest and is quite polite to them. Later she hears a news report that some aliens visited earth and thought that humans are good enough that they do not need to be exterminated! We then find out that the nice men were the aliens. Their skin is green, but the woman is colorblind (literally) so she did not notice. Phew!
According to the ISFDB, the story first appeared in The Magazine of Fantasy and Science Fiction in February 1952, and has been anthologized several times, such as in The Science Fiction Century edited by David G. Hartwell (yes, that's where I read it).