From the Greek text, it looks like this means "I have a plate with oranges," in the same sense as "I have a fruit plate" (or platter). But this could just as easily mean "I have a plate that is the color orange." Would this be "ένα πιάτο πορτοκάλι?"
I'm confused now. Maybe this sentence shouldn't be used in the skill about Food. I'll see if it can be changed. But in the meantime, to answer your question "Εγώ έχω ένα πορτοκαλί πιάτο" means that I have a plate that is the color orange. "Α plate with oranges" would be "ένα πιάτο με πορτοκάλια"