Can you get red greengages?

I have discovered about 6 huge trees in a local park, where they are dropping wee like plums.... I have managed to scoop up about 30LB of these and made some fab jam and pies.... I dont know whether they are greengages or not. They are small and vary from a very light greeny yellow to a dark red.. any ideas if you can get red gages or shed any light as to what they are many thanks