What is the reason that the blue color is hard to come off after half an hour of dyeing glue with this dye solution? How to solve it?

What is the reason that the blue color is hard to come off after half an hour of dyeing glue with this dye solution? How to solve it?

It is possible that the impurities in the gel have not been removed, causing the difficulty in removing the color, which can be improved by replacing the deionized water at the same time during the process of removing the color, prolonging the time of removing the color and increasing the temperature of removing the color appropriately.