Buckthorn has relatively decent lightfastness on wool, and many shades can be obtained if you use fresh leaves, berries or bark. 

Dried buckthorn bark gives yellows and with the addition of an iron afterbath, olive green. Freshly harvested buckthorn bark will give earthy light to deep pinks and rusty reds, so I’ve seen or red, but have not tried myself. The bark contains tannins and I’ve read that no other mordant is needed, but perhaps the use of a mordant would increase lightfastness.

Buckthorn Extract is made from the green berries and gives vibrant golden yellows or vivid olive greens if you use an after bath of iron.  Fresh leaves produce a bright yellow, increasing in intensity with the amount used. 

Buckthorn berries turn from green to red to dark blue/black.  Fully ripe berries can give a green on wool, and the berries could be frozen and use later in the year.  See this article online from Spin-off magazine, for information on possibly obtaining blue from fresh buckthorn berry skins. 

The above online article, also shares an alkaline fermentation bath for achieving the earthy pinks and reds, by putting the bark in water, and adding washing soda to get a pH of 11-13.  Apparently, with no heat, the alkaline conditions do not harm the wool, like it would if heated.  The color is extracted from the bark after being left to sit with the wool in a cool place. 

I should try the alkaline fermentation bath on wool with this dried buckthorn bark, to see if I can obtain the pinks and reds, that were obtained with fresh bark. 

Buckthorn is a shrub found in North America, and is considered invasive, but has been used historically for pigments, paints and inks.  There are two species:  The first is Rhamnus frangula (Frangula alnus) known as glossy buckthorn, alder buckthorn, or black dogwood. The second is Rhamnus cathartica, known as common, European, or purging buckthorn.