With eco or contact printing as I aim to explore it, I am not first concerned with dye standards related to the needed longevity of wearable textiles. I am making art textiles and as such, my art is subject to normal conservation practice, the key practices being: Keep the art out of direct sunlight; rotate the art on walls etc seasonally if possible; clean the art with a gentle suction vaccuum through a piece of window screening; store in a dark place in an acid-free, dust-excluding container. Dry cleaning can work for some textile art works.
So with that in mind, here is my approach to fastness of colour when eco printing with Red Cabbage. My understanding is that the natural dyes present in textiles coloured with Red Cabbage will NOT withstand regular, normal laundering and will NOT be lightfast if exposed to sunlight a lot. Paula Burch (master dyer at http://www.pburch.net) maintains that no mordant on earth will render permanent the colours obtained from Red Cabbage. This caveat may apply also to many other natural dyes used for colouring wearables, particularly if larger amounts of fibres are dyed and if long lasting colour, even colouration and reasonable standards of replication are your goals as a dye artist.
What that all means is that I am not going to be suprised if my eco prints with Red Cabbage or other plants fade. Only time and experiments will tell. Meantime, I am going to do my best to make sure they last by following the Best Practices of experienced dyers, where Best Practices do not prevent experiments. And even, as India Flint proposes, consider re-dyeing or re-printing.
Now for some more experiments with Red Cabbage.
Three jars with red cabbage and one cup (250 ml) of hot tap water in each. The blue colour came out immediately.
Left, blue, water only; centre, violet, water and 25% white vinegar; right, green, water and 25% household ammonia.