Hello POTP -
Thanks for bringing this to our attention.
Based on what you have described here, it sounds like the issue you've run into is on the Saved Listings page as opposed to the Inventory Management area.
We took a look into this and it seems that currently if your Post Threshold and Stock Threshold are set to 0 and if the quantity is 1, the button will be yellow, which is appropriate. However, if the quantity reaches 0, it shows as red on the Inventory management page correctly, but the button is staying yellow on the Saved Listings page instead of going to red at that time. We have a work ticket created regarding this issue and I will update this thread as soon as more information regarding this issue becomes available.
If you are seeing this differently in your account, please file a support case letting us know the details of what you run into, where you are seeing it and a few examples from within your account so we can look into this further.
Please note that you can currently work around this by setting the Stock Threshold count set to 1 instead of 0 you should see the appropriate symbol on the saved listings page.