when i change the post body font size using this code;
body.single-post div.post {font-size: 13px; line-height: 1.3px; !important;}
the font color also changes. How do i change only the size of the font without affecting the original font color?

That code should not effect the colot at all - not knowinng where you put it or the url of the site there is no way to check if you might have made a tyopo or removed the color rule that was there.

go back and recheck where you put that code and what was there before
use something like FireBug in firefox to examine the css being applied