Problem Solved in a convoluted way.
I moved the entire files rather than the snippets containing the colour I wished to change to:
/modules/wall/templates/tmpl_mytemplate/css with the following file structure:
outline.css (completely moved and deleted from /templates/base/css)
view.css (ditto)
I found that by leaving a copy of the files in the /base/css folder, I was still getting the problems mentioned in my previous post. However, by deleting the files in /base/css, it's necessary to create a folder tmpl_whatever_template for every template you use and include the css. Of course you can now change colours as you wish.
Until Deano finds an easier way, this works.