The way I would approach this is to use a calculated field on the (I assume) conversion percentage and use an if statement:
if conversion % > (reference line) then "green"
elseif conversion % < (reference line) then "red"
Add this to the colour shelf on your bar chart (use multiple marks if necessary) and then you can change your colour to 'red' and 'green' if necessary.
In the tutorial video on Box and Whisker charts, the tutor shows how to create control charts and the above method is used.
Hope that's clear,
Thanks for your reply. I'll have a look at the tutorials to see how it is done. Thanks for your help.