This style for Google Calendar gives the gadget headers a background color gradient--blue by default (see comments in the code for other colors, or choose your own) and colors/centers the text (as you like). (For a style that makes it [hopefully] easier for you to pick your own colors, see http://freestyler.ws/style/29542/gcal-gadget-header-gradient-color-hsl.)

You could also use a background color in addition to the gradient for more variety. This image shows a few colors which have been used along with the charcoal gradient (red, orange, yellow, lime, cyan, blue, purple, magenta):
http://a.imageshack.us/img827/9999/gcalgradientcharcoalroy.png
(use http://freestyler.ws/style/29648/imageshack-img-centered-w-dark-bg for a "better" view)

Update...
31Jul2010: added hover/click values for charcoal (commented out--sample)
28Jul2010: added hover/click effects (for the default blue only [as a sample--other colors would require a similar color code change to the base, typically about 5% to 10% lighter]); added text-shadow (commented out--optional, to use as you like; the image below shows the default text on the left and with text shadow enabled on the right)