Contact form 7 uses the wpcf7-form class for it’s forms and wpcf7 for it’s parent DIV. You can set the color of the input types by these CSS rules:
.wpcf7-form input[type="text"],
.wpcf7-form input[type="url"],
.wpcf7-form input[type="password"],
.wpcf7-form input[type="email"],
.wpcf7-form textarea {
color:white !important
}
The !important rule will make it override any other style’s rules. Head over to Appearance > Customize and add the above rules to your custom CSS.
PS : You can also use .wpcf7 input[type="text"] if your form’s class is different for any reason.