text in Reminders changed color

Ah, I see. The list title. Do you see the sidebar on the left of Reminders? If not, choose View, Show Sidebar.

Then, in the sidebar, hold the Control key and click on the name of that list. In the context menu that appears, you should see some color chips. Choose the color you want for the list title.

Do you mean in one single Reminder item?

Select the text in that item. Then Control+click it. Select Font, Show Colors. Set the color of the selected text there.