Hi Paul,
The situation you described actually does not involve the Auctiva Inventory system like the one described by the original poster, but I was able to easily reproduce this behavior and I believe it is expected based on how eBay handles the display of the quantity available and quantity sold for variation listings.
It appears that eBay updates the quantity available that is displayed dynamically based on which variation is selected (Colour in your case) and always displays the number of items sold from the listing regardless of colour as the number of items sold.
Although the variations that you enter when creating listings like this through Auctiva are submitted to eBay by our system, we do not actually have any role in how these values are updated on eBay as purchases are made from them.
If you feel that the quantity sold for these listings should update dynamically based on which variation is selected, you may want to contact eBay's Customer Support team to make them aware of your feelings - even if just as a suggestion.
-Mike