I have generally found that the purple versions of things are more robust. The carrots are ready sooner, and less likely to get all neurotic about soil conditions (within reason, I mean they are carrots after all), though they do seem to be hairier (ie more roots - maybe that's why). The purple shallots have survived where as the wussy green ones didn't even try (well, this may have something to do with where they were planted and where the wretched chooks chose to go worm hunting - but yanno, details). And the purple beans are more prolific - and also awesome when you cook them and they go green. 'Mazing that.
I'm feeling a bit of a purple summer coming on actually. Have been fossicking about on Diggers and looking at all the purple versions of artichokes, basil, beans, eggplant, passionfruit, garlic... I reckon I could declare it the Year of Purple and plant purple everythings. That could be so much fun (well, only a gardener would understand the funness of it, the rest of you will have to humour me).