The answer: by using hiccup's Sample Skin I found that I was colouring the wrong scroll bar doh! Again the Sample skin has come to my rescue - thanks hiccup.
A small rectangle in the background colour appears on the scroll bar of the Artists & Juke Box main panel views (outlined on the right hand scroll bar). All other skinnable scrollbars do not display this artefact (see scroll bar on the right);
Question: can I get rid of it and if so how?