You can ad custom color sets to your copy of the Garland theme by editing the file color.inc located under your sites/all/themes/yourgarland/color directory.

To obtain the appropiate set, you can first select the values using the custom option and use the values later on the file. A very good way to get an approximation of the correct colors is to extract them from a picture that you feel is representative and use a resource like the Color Palette Generator to extract the main colors.