The problem is that it all depends...
What cloth you are dyeing ?
What you are dyeing it with ?
What mordant you intend to use, or not ?
Have you got a big enough dyebath to move the cloth freely around so that it dyes evenly ?
How hard or soft is your water ?
Where have the plants been grown and when were they picked ?
If you still want to have a go then wool will have the best take up of colour. Alum or Copper mordant (Iron can give you mud colour with them) will give the best results with Elderberries, and if you let them ferment before you use them you will get more colour from the dye liquor ( the fermentation breaks open the colour in the skins, think how staining red wine can be).
Most berry dyes are acid/ alkali sensitive. If you use acid, like vinegar, you will get a pinky type colour. If you add an alkali, like soap or washing soda, you will get greyish blues.
It's interesting though to see what you get. Let us know ?
cheers,
Toddy