The margins of the fruit edges are definitely yellow once fully ripe on the tree. I would assume store bought under ripe picked ones are fine for the recipe too though. Often times not every fruit we process is actually tree ripened.
I'm sure some of the oxalic acid is broken down in cooking as with other high oxalic acid foods. Beyond just the initial cooking they then go through a water bath canning as well so the jelly once fully done is very cooked.
I've probably eaten 5-10 whole fresh fruit in a sitting without any I'll effects. I've researched the whole oxalate health issues thing and personally don't believe it's much a big deal unless perhaps you have underlying conditions. A normal healthy person I don't think is greatly affected by moderate to low amounts of oxalates. You probably wouldn't want to eat copious amounts of high oxalate material either way you slice it but keeping that in mind and moderating your intake would subvert any major issues. Find it funny when people are so worried about eating a raw spinach salad once and awhile but they chug down a mountain dew which in my opinion is pure metabolic poison. Be afraid of that slightly toxic natural substance(that a healthy body can totally deal with in considerable amount) but take in preservatives, high fructose corn syrup, artificial dyes and chemicals, etc, etc without a question.