Hi
The most common cause of 'greening' in garlic is the bulb starting to grow.
Old processed garlic usually goes yellow and generates an acidic smell.
I have in my time seen vivid green garlic, almost fluorescent, but the product is safe, if a little 'surprising'.
Whether it goes green depends on when it was harvested, how long, conditions and temperature it has been stored at.
In my experience Chinese garlic has been the most susceptible to problems including going 'green'.
The easiest test is get some bulbs from the centre of the store.
Garlic can produce some heat if not properly stored, so test from the centre.
Cut the bulbs in half and check the colour.
There is any trace of green in the centre then there is a strong possibility that the final paste will be green or have a tinge.
If its white throughout you should be ok.
This is not a spoilage or oxidation issue it is a plant growth issue so preservatives don't really help.
Remember that garlic bulbs are exactly that and their raison d'être is to grow and produce more garlic plants, which they will do enthusiastically given the opportunity.
Hope this helps.