OK. your need is obviously great!
Choosing: colour depends on type of mango, and where it's from - and I don't mean Sainsbury or Tesco.
Basically, you need to pick up the mango, sniff it (if it smells all lovely and fragrant, it's probably ripe, even though it might look green and unripe), then gently press the fatter end with your thumb. If it yield to slight pressure, it's likely to be nice. It can be any colour.
Secondly, read the label before you buy, depending on whether you want a smooth, creamy-ish flesh, or a more fibrous one. I think (personally), the smooth ones are best for eating raw and the fibrous ones are better if you want to cook with them, in a curry or a chutney, for example. It usually says 'fibrous' or smooth. Well, it does where I buy them.
Cutting and eating: Use a very sharp knife. Stand the mango on a board on one of its long sides and poke the knife in at about the middle. If you hit resistance, that's the stone. Move the knife over a bit and stick it in again, repeating til you ge to a bit that doesn't hit resistance. Then just slice down the side of the stone in one movement. Repeat on the other side. Mango stones are rather flat and elongated and fill up quite a lot of the interior. You should end up with two mango 'cheeks' and a stone with some flesh left on it, surrounded by skin.
Your choice is then, peel the cheeks, cut them into slices or dices and do whatever you want with them or you can cup each unpeeled cheek in your hand (skin side down) and gently mark the flesh into squares. Once you've done it, you can press gently on the skin side and sort of turn the cheek inside out - mango hedgehog.
the stone bit can be peeled and the flesh just cut off any old way you like, or sucked in the bath.
I've just read this back to myself. Sorry - it all sounds a bit porny, but that's mangoes for you..................! Enjoy!