I looked into what it would take to make a macro and it gets pretty involved
since column headings have their own cells and empty days have their own
cells, etc. But doing it by hand doesn't seem all that involved as long as
you do one color at a time since the font color chooser stays on the last
choice. So for instance, after selecting the first day and making it red,
select the fourth day and click on the color, select the seventh day and
click the color, etc. Then do the blue for the whole year.  And so on.  I did
this for one month and it took 2 1/2 minutes.  If you did it with styles (as
per Mary's suggestion) i.e. day1style, day2style, day3style it would take a
little longer but you would have the advantage of being able to change the
colors by modifying the style.

I'm not sure if I'm understanding you correctly, but could you use the format
painter to quickly copy attributes from one day to another? If you double
click on it, it lets you click multiple times in your document to easily
change text that isn't all grouped together (like days of the week or titles
in a newsletter). It's still being done by hand, but it's a little quicker
than what it sounds like you're doing now.