1. Consider a dictionary with several keys, the best way to select the keys you want to study is to create a switch for each of key. Use determine statements to select. A little bothering in the begining, but much beneficial in the long run.
for matplotlib scatter function and aplpy show_markers function, the argument c can be a n*4 array which specify the rgb alpha values of each point, while argument color, edgecolor and facecolor can only be scalars or 1D arrays.