Hi Luke. Walter from microDev here (Shopify Partner).
what you want to do is go to the “Theme” option in your Shopify Admin online store, and press the “Customize” button. Then on the left hand side, the list of sections on your home page will be displayed. Click on the “Header” section and then find the option near the top which says “Links and icons”. The colour selected there is what determines the colour of text in your navigation bar.
If you need any further support or help with customisations of your website, please feel free to intact me at firstname.lastname@example.org.
All the best!