I hope you are well today and thank you for your question.
Please make sure that you have configured a search form in the Menu Search options as shown in the below screenshot.
View post on imgur.com
Then you can change the text color using the Search Form Customizer displayed on the below path.
Admin Area -> Ivory Search -> Search Forms -> Edit Search Form -> Design -> Search Form Customizer
Alternatively, you can achieve this by adding the below CSS code in the Custom CSS option of Ivory Search plugin settings on the path Admin Area -> Ivory Search -> Settings -> Settings
#site-header input.is-search-input {
color: #000;
}
Best regards,
Ivory Search Support