The two words are both a light purple and are named for types of flowers. Here are some links to demonstrate: http://en.wikipedia.org/wiki/File:Lilac2008_by_VictorLee.jpg http://en.wikipedia.org/wiki/File:Topped_lavender.jpg The difference between these two hues is too subtle for there to be a standardly defined separation. In other words, these words are assigned primarily by the opinion of whoever dyed the fabric.

If it helps any, lilac and lavender are both flowers. So for example, you could translate "lilac" as "the colour of lilac flowers". It's a bit wordy, but very clear. I don't know enough Chinese to know how names of subtle colours are adopted. -__-