If there were a choice of colours how would you say "the purple (colour)" ? Because "the purple" is not accepted for "a lila" here
I am not sure I understand your question exactly but I think "... the purple" should (also) be accepted. When we are talking about colours as things, it is "a lila". When we use them to describe the colour of other things, "lila".
"A hatodik tojás lila."