I would like to be able to change the color of the days once they're over - is that possible?
If you switch to Google Calendar's "new look", past events will be faded. If you are putting off the transition because of the extra white space, just like in GMail you can reduce it by clicking the cog wheel right above the calendar and selecting Cozy or Compact.
The interface is designed to highlight only the current day because that's what requires the most attention. If you try to change the background of past days, you'll have to change the whole background so that they don't become more visually prominent. That's why the "new look" fades only the past events instead of entire days.