Dear Farhad,

Clearly the colorant you use must firstly not cause your experimental micro-ingredients to precipitate and secondly, not precipitate itself. Methylene blue and most food dyes carry a positive (or cationic) charge in solution and this is a key factor both in their solubility in water and to loss of solubility by chemical reaction with negatively charged anions. You could try using your colorant in acetone or alcoholic solution or by adding a cationic or non-ionic polyol surfactant.

Dear David Oulton ,

Thank you very much for your useful information.