Are you talking about the mangoes that turn yellow when ripe like champagne mangoes that happen to be unripe or the ones that turn red/ yellow when ripe? Either way you can blend it with Pectinex. I've always done 2% by weight of flesh and blend it together. Once blended you can seal it in a jar or bag, use a sous vide at 122 degrees Fahrenheit or 50 degrees Celsius for an hour. Pectinex works best at that temperature. You can alternatively put it in the fridge overnight and strain it the next day, but sous vide is faster. Or you can put it on the stove in a water bath, but you have to monitor the temperature carefully so you don't kill the enzyme beforeor finishes the job. Once strained through a coffee filter or layers of cheese cloth, you can taste it and either acid adjust it or add sugar to turn it into a syrup. As for the peels, I've turned it into tepache. Just check it every day so it doesn't over ferment. I've used the clarified raw mango juice as an acidic base for cocktails, I've also added champagne vinegar and sugar to make a shrub.