If nobody minds, I'm gonna pop this into this old post (so I know it's recorded somewhere for future!

).
As Van has already pointed out, the cell colour before and after rollover is controlled in the theme.php and in the css.
The actual colour displayed
on rollover is a different matter. This is controlled in the jcal/includes/functions.inc.php file, utilising the colour settings from the above files.
function colorHighlight($hexColor) {
// highlights a color by increasing it's luminosity
$temp = hexdec(substr($hexColor, 1)) - hexdec("140A04");
return "#".dechex($temp);
}
So, taking these defaults as a starting point, what this calculation effectively does is subtract the hex colour value #140A04 from #ffffff (white) to leave you with a blue-ish rollover effect. All you need to do to gain complete mastery over the calendar cell's rollover effects is a little bit of hex math! Or, use this handy little calculator I found at
http://www.guycroft.clara.net/utils/coloropts.html. Just put your desired rollover colour in the top 3 boxes (split the hex into 3 chunks) and click the "Complementary" button. Enter the generated complementary hex code into the functions.inc "hexdec..." code quoted above (replacing 140A04) and Bingo!
John Mc