How Cani keep greengage jam green rather than orangey brown?
It sounds like it is oxidizing so you need an antioxidant for the jam. First I need to know what ingredients Greengage jam contains. Then I can advise you on natural antioxidants to add (like lemon juice for avocados).
The only way to keep the the green color you want would be to use a no-cook (freezer) recipe to make your jam. The process of cooking breaks down the chlorophyll in the plums, eventually changing it to a yellowish color.
For the geeky science explanation you can check this out The Why, How and Consequences of cooking our food