The basic answer is that you'll need to look at what styles are defining those colors and then override them. This typically involves using the Firebug extension to Firefox in order to inspect the elements on the page.
For example, inspecting the Month button on the calendar reveals that
.RadScheduler_Web20 .rsHeader .rsSelected, .RadScheduler_Web20 .rsHeader ul a:hover is defining a background color and image to produce that green color. You can override this by defining a more specific selector, like .RadScheduler_Web20 .rsHeader li.rsSelected, .RadScheduler_Web20 li.rsHeader ul a:hover (notice how I added the element type, li, to the final class for both selectors). Using this selector in your portal or stylesheet, you can define a new background color and/or image for that section of the calendar.
Hope that helps,