Any fan of a franchise will know not everything gets translated from page to screen, especially minor details that casual fans hardly know about. Consider yourself lucky when show creators simply change something like a character's hair color instead of a major plot point. That being said, it's easy not to fault Game of Thrones showrunners for not featuring Daenerys Targaryen's violet eyes.
In George RR Martin's books series, A Song of Ice and Fire, the Targaryen family are known for their silvery white hair and violet eyes. So when it came time for the first episodes to be shot of the TV adaptation, the creators actually did make Emilia Clarke, who plays Dany, and Harry Lloyd, who played Viserys, wear violet contact lenses along with their iconic white wigs at first.
So what happened to the purple eyes, you ask? They got canned, because showrunners David Benioff and Dan Weiss apparently thought that the lenses wouldn't allow the characters to show emotions correctly. "Actors act with their eyes, and [the lenses] really hurt the emotion," they stated in one of the Thrones DVD commentaries, according to Den of Geek.
So it seems if the lenses had stayed, we wouldn't have gotten some of the great, emotional scenes from Daenerys.