Why only methylene blue, hydrochloric acid, metanil yellow are used. What is the principle behind this test

Primary objective To remove the different types of husk around a plant material consisting of any
kind of husked grain/pea, e.g. immature wheat grain. It replaces a classic combine
harvester when the water content of the material is too high and this consequently
could turn into a paste. (1,2)
Working principle The whole operation is a wet process performed using a specific device which is
described below:
The plant material is introduced into a fluid (e.g. air or water); then that fluid
loaded with the material is projected (e.g. pressurized water through a nozzle) at a
specific angle on at least one rigid plate, the impact of the products mixed with the
fluid causing removal of the husk(s) (i.e. glumes and glumellae for wheat)
surrounding the vegetable material. The grains are separated from the husk(s)
(e.g. in a settling tank, using a cyclone, etc.) The selectivity rate of this technology
is high: 985 out of 1000 grains are successfully dehusked. The dehusked material
should be decontaminated and/or dryed for a proper preservation. (1,3)