Changing Font Color in Ninja Form

Please provide a link to a page on your site where we can see this. Thanks.

Also…

The developers and users of that plugin would be the best people to ask. You can reach them here:

https://wordpress.org/support/plugin/ninja-forms

I think this CSS may do it

input.ninja-forms-field.
textarea.ninja-forms-field {
  color: #000;
}

To add CSS: If you are using WordPress 4.7, use the “Custom CSS” option in the customizer. If your theme has a custom CSS option, use that to add the CSS shown above. If not, install the plugin Simple Custom CSS. Or, if you have Jetpack installed, enable its Custom CSS module.

Learn the Chrome Developer Tools to help you see and test changes to your CSS.

That should be a comma after field, not a period. If you copied/pasted, try fixing that. If it doesn’t help, check with Ninja forms support. There’s probably an option somewhere to set colors.