orchid questions

[Little One]

What causes a healthy orchid plant that sends up a new growth to have the tip of the growth come up black and the rest of it a healthy green? Also a second question, on another orchid plant, very healthy, sending out new growths, with all the promises of a potential bud-- to suddenly overnight have the
sheaf turn yellow? Not much temperature variation involved. Can someone have some answers for me?? Thanks to any and all.