One of the new features available with fieldTask version 4 is the ability to add color and other styles to forms.  This feature was contributed to odk by Nafundi and you can find out more on their website.  https://nafundi.com/blog/posts/customize-odk-forms-with-fonts-colors-sizes-and-links/

If you want to try it out there is an example form, also created by Nafundi although I have made some minor changes to it.