I'd rather not have the fallback applied whilst using the radial gradient mixin in Bourbon.
The documentation http://bourbon.io/docs/#radial-gradient says it's optional, but whatever I try, the primary colour always gets applied (I've tried entering false, null or none as $fallback).
Any tip greatly appreciated. Thanks!
Looking through the radial-gradient mixin, I believe that the fallback value allows you to specify either a colour or "transparent". If you don't specify a fallback it uses the first colour that you provide it.
To your question, I believe specifying transparent against the fallback should solve your problem.
For example: