Why are the red potatoes so bitter

I used to cook plenty of purple sweet potatoes when I was living in africa:) Sounds like you maybe cooked them too long...

They can be extremely tasty, sweet with a "perfumy" hint if you are lucky. They are best boiled with the skin on like you did and peeled afterwards (as the taste is preserved better that way). As for cooking time it really depends on the size (which means the potatoes are likely done at different times) but you can always feel when they are done by pressing on the potato with a fork - it should give slightly/be soft. I think it usually took me about 30min but I didn't use a clock. It may shorten the cooking time slightly to make holes in the potatoes with a fork (this also adds to the "creaminess").

I guess they do sometimes get a softer, more "creamy" consistency than your usual potato when well done. However the taste is richer and in my opinion far superior to orange sweet potato. If you prefer a firmer potato, just let your potatoes stay on the firm side (in my experience, though, the taste is enhanced by letting them go soft). But boiled ones would anyways mainly be for puddings as it gets rather sweet.

For panfrying I would suggest avoiding boiling the sweet pots first. Simply peel/wedge them raw, fry them as chips until done and serve with a spicy/hot dip - yum.

The only time I've ever heard of something similar is Salt Potatoes

www.nytimes.com/2008/08/22/travel/escapes/22rNYfood.html

allrecipes.com/recipe/syracuse-salt-potatoes/

en.wikipedia.org/wiki/Salt_potatoes

but I've never seen them done in a pressure cooker. However, most pressure cooking charts (http://fastcooking.ca/pressure_cookers/cooking_times_pressure_cooker.php for example) list the cooking time of whole new potatoes at about 5-8 minutes. If it was me I would try one of the salt potato recipes and just cook them for less time.