I have occasionally worked with carotenoids myself, but your specific application is new to me. Methanol is a possibility, although it is pretty polar. However, it seems to me that if you must have an organic solvent present, you might want to learn the maximum solubility of astaxanthin in that solvent ahead of time and then make the solution as concentrated as possible, so as to minimize the volume of solvent that you add. There is a lipid database (hosted somewhere in Japan) that has a great deal of information about carotenoids on-line. I have not looked for solubility information there, however.