The methyl violet 2B power that I have has a dye content of 82%. I am going to scan roots by using Winrhizo, so just want to have a sharp contrast with the background.
If you use 5% of Metil you can attain a good contrast to scan and use winrhizo, but another important factor is the time you put roots in contact with the dye. Yo must do a cinetic curve to find the appropiate time to submerge roots in the dye. Surely it will take 15 minutes, but it depends on the type of root tissue.