Stuart -
I was able to finally get all of my variants to sync to google by assigning each color/size combo a unique image file (even though the actual image was exactly the same for each color). This had the major downside of cluttering up my product page though.
It seems that with Shopify, it isn't currently possible to have a clean looking product page and have all of your variatns sent to google.
I've been very surprised by how unhelpful the Shopify support has been throught this process. I can't imagine I'm the first customer to come across this issue.