What prevents the cell from appearing green?

 if this is about endospore stain, the answer is: Vegetative cells are easily decolorized so the green dye washes off

Carbon-14 has a half-life of 5,730 years and most igneous rock is older than that.

You should never redesign your experiment. try and see what in your experiment you made a mistake on or try and test your experimemt with somebody else. see if they get the right results.