jQuery and colour calculation

Question

Currently I've got an H1 tag set to a colour of #8c0000 (a deep red).

Using jQuery, I would like to get the colour of the H1 and then do a calculation based on the h1's colour to determine what the hex value of a new colour would be if I wanted it several shades darker.

The reason for this is that I want to use CSS3's new text-shadow property to create an "embossed" effect by creating an "inset" text shadow.

To get the H1 elements colour, I believe, can be done by using:

('H1').css('color');

But what do I do with that value to calculate a darker shade?

PS - seeing as the H1 color will be set dynamically, I can't just hardcode in a fixed text shadow, which is why I need the calculation...

Any help would be GREATLY appreciated. Thank you in advance