What is your theme? Ambiance?
Are you using gtk2? or gtk3?
Have you installed classic Desktop or you are using unity?
You can install gnome-color-chooser
sudo apt-get install gnome-color-chooser
I have used this upto ubuntu 12.04, it should be worked on ubuntu 13.10 too.
When you open it, there is a tab name 'specific'. Under scroll bars section, you can change color of scroll bar. Click on Apply.