Who picks the MUNI pass colors?

I have become completely obsessed with this urban mystery. At the start of each month, I dutifully head over to Safeway to pick up my MUNI Fast Pass. The best part of this routine is seeing what colors have been selected for the month. They always seem oddly seasonally-appropriate to me. For example, December was red and green and April was bright blue and yellow.

Plus, I'm not the only one who appreciates this extra effort. There is a whole art project that features more than 2,000 expired MUNI passes.

So, who is this mysterious color decider? Is it some bureaucratic minion expressing their secret artistic nature? Does MUNI have a highly-paid graphic designer who selects the colors? Is it a completely random process that just appears to be conscious? Is it Gavin Newsom? Does one lucky bus driver get to pick each month?

Does anyone know how this really works? C'mon Chronicle, I need to know!