Settings of the Contact form > Placeholder > Text color https://prnt.sc/v4a4xn
or use the next custom CSS code – https://prnt.sc/v4a53j (Theme Options > theme Custom CSS or custom CSS for page or section, or element area)
.etheme-contact-form-7 .wpcf7-form-control.wpcf7-textarea::placeholder {
color: red !important;
}
