They did. As far as I remember Balearic islanders and other Iberian peoples used a mixture of indigo and madder to create a form of purple. However, it was far inferior to Tyrian purple, which, apparently created a dye that was truly beautiful. I don't have the reference to hand, but I can remember reading a description of it in a text about the Seleucid kings, "Alexander to Actium", I think, which describes it as shimmering. http://community.middlebury.edu/~harris/Classics/purple.html

Actually, it's a situation similar to modern day pearls. Now a days, the only real way to differentiate real pearls from fakes is to see if they are tied individually or not, this avoids all of them flying out when the necklace breaks.

Back in the day it was the same, you could obtain the same colour, but only the one obtained through the traditional and far more expensive methods was considered valuable, though the "real" tyrian purple is said to improve in colour instead of decaying with time. Someone also asked if it was true that clothes dyed this way also smelled terrible. They did, and the smell, way harder to replicate, was often used as a way to know if it was "real" tyrian purple, this smell was produced by the fact that the snails used to produce the dye had to decompose first.

To add to OP's question: is it true that clothing dyed with Tyrian purple smelled terrible?