6-OHDopamine is not very stable, so that you will need to prepare fresh solutions. As lower pH, greater stability, so that above pH 6.5 you will have darkening quite fast. So, you have to thinkg about the pH of the cell culture media. 0.1 mM EDTA and degassing solutions increase 6OHDA stability. Good luck 

when you say prepare fresh solutions, do you mean you weigh out how many grams you need and dilute it each time? But usually 6 ohda is sold in a 5mg to 50mg bottle, isnt it difficult to weight out how much you need each time?

Thanks

Thomas