Note: Please ensure that you have access to an experienced JavaScript developer, because Marketo Technical Support is not set up to assist with troubleshooting custom JavaScript.
Currently the form editor only allows you to change the color of the input text, not the text labels. You can, however, drag a “custom html” block into the landing page hosting the form and use the following code as a solution:
<style type='text/css'>
form.lpeRegForm ul { color:#FFFFFF !important; }
</style>
Replace the bolded section with the color code of your choice.