Changing color of Thank You message for DIVI Contact form

To do this, add a custom CSS class to that particular Module Settings / Advanced / CSS ID & Classes say custom-contact, then add the following code to Dashboard / Appearance / Customize / Additional CSS:

.my-new-contact .et-pb-contact-message {
color: #333;

You can play with the value to achieve the desired result.
Let me know the result.