Okay so i was thinking really hard about haki and i have come up after a lot of research that it is possbile but not the degree in one piece.
Colour of observation- this is the easiest haki to do put requires your senses of touch and hearing to enhance because the only way to train it will be basically you blindfolded and dogding something anything such as a tennis ball shooting machine or like in one piece when luffy was blindfolded and rayelige was hiting him with a baseball bat but ofcourse dont go that crazy luffy was protected to a degree because raylege wasnt using haki.
Invisible amror- Very complcated and kinda impossible but you can strengthen you body i.e. training it to not feel pain like mma fighters do and to hurt a picec of body multi times so it will not hurt i.e. strengthen it also when luffy uses his steel hand it is possbile i think they call it in karate the iron fist or sholain monks use it by training their hand to strengthen so strong as steel but will take forever because they somehow use their strength by usuing the cells in their hands to harden like while in an adreline rush it blocks about 50% of pain i think.
Kings haki- is the rarest and hardest because it requires your strength of producing fear at people by using fear or will and image i.e. look strong and scary , have a scary reputation or to make people go mad or faint scare them by image by somehow changing into a unimaginanble scary form which is ofcourse impossible but it is like your aura of fear your pressense makes the air around a person scared and it is weaker thats why when you scare them even more their air gets cut off and the faint but not like drowining lol