Go to your specific Live Editor >> click on Page Setting >> Other Scripts >> Custom CSS >> insert the code below:
.optin-box-1 input{
color:#000 !important;
}
Replace the number of the opt in box class with the ones that you are using. The first opt in style in our list is 1, second opt in style is 2 and so on.
Feel free to use your own input text color. You can find the html color code here.