All fish red cells have a nucleus. Acridine orange usually stains DNA green but if you use too much dye per cell it will stack up and produce a metachromatic shift in its emission spectrum. This is used to advantage in mammalian cells where low pH in micro-vesicles cause the same phenomenon. Thus you get green nuclei and red acidic vesicles. If you image with too much excitation light you can also change the emission wavelength of AO. Getting the correct concentration of AO is quite difficult as it is not particularly soluble in aqueous buffers.

Thank you so much Sir.