We picked a load of wild plums the other day which were yellow with hardly any pink on them. Shouldn't have picked them because they were not quite ripe - I beg to differ as you can easily ripen them off with the aid of a couple of bananas on a covered tray as per the photos below.
They are now nearly all a lovely pink colour. I think the jam pan is going on shortly to process some of natures bounty into home made wild plum preserve. You can use the same method with ordinary plums, or to get tomatoes to ripen.
And the fruit its absolutely delicious a completely underrated fruit.