HoudahSpot 2.1.4 added a hidden preference allowing for the accent color to be customized. Enter one of the following at the command line in Terminal.app and relaunch HoudahSpot.
defaults write com.houdah.HoudahSpot accentColor yellow
defaults write com.houdah.HoudahSpot accentColor red
defaults write com.houdah.HoudahSpot accentColor green
defaults write com.houdah.HoudahSpot accentColor blue
defaults write com.houdah.HoudahSpot accentColor grey