Yes that is right, no chemicals, just remember though that a lot of plant matter is required to obtain colour and that natural dying by this method will yield soft colours only. To obtain a stronger colour you must over dye several times letting each step dry between dying and heat setting. You can buy from your local craft store 'Mordant' which gives a better dye set but there are several kinds and I would not be able to tell you the content obviously, some are quite harmless like wood ash and so on so it may pay to inquire what is available near you. Your library or on line might give some recipes, some plants give beautiful yellows but one needs to know when and where to harvest etc, gathering plants in the wild is so time consuming no wonder folks opted for chemical dyes when they became available along with mechanization!
Gal