However, the white under the arm and around the high collar have gone yellow how do I clean these white sequins? Please help.
A: Thanks for your note! If the sequins themselves have turned color, there is unfortunately no way to clean them. When a plastic turns color, it's due to chemical reactions between the plastic and the surrounding environment, probably due to perspiration as it's at the underarm and at the top of the collar.
If you attempt to change the color of a plastic sequin, it will possibly become brittle or melt to the fabric nearby. You could attempt a dry cleaning to solve this, but it would not be a likely solution.
The best solution I would recommend is to find a match to these sequins, purchase a quantity of them, remove the discolored ones and replace with new ones. You would also need to stabilize the sequins around the work area because there will be loose ends to their securing threads once the others are cut.