Yellow is the most common natural dye colour. It would be impossible to list all of the plants that make yellow (but don't think I don't try in the Yellow section of this website).
Yellows are "direct dyes", that is you heat them in water and the water becomes a dye you can directly dye your fibre with.
The part of the plant used depends on the plant.
Plants that produce yellow vary in the amount of dye stuff they contain. It often requires an equal amount of plant material as there is fibre to be dyed to get a strong yellow (100% weight of fibre).
"There is no blue without yellow and without orange..." van Gogh 1888.