how to modify formData jquery

I want to submit a form with ajax. I get data of from using FormData like this:

var data = new FormData($(this)[0]);

one of form's input is a color value in HSV format. I want to convert it to hex value. I have a jquery function to convert it, but I don't know how can I manipulate it's value in FormData. help me please!

another question is: How can I remove some fields (key, value) from FormData?