I'm changing the colour of the header and text. What do those letters (FFFFF) mean?
These are HTML colour values, HTML colors are defined using a hexadecimal notation (HEX) for the combination of Red, Green, and Blue color values (RGB). You don't need to worry too much about them as you can click on the selection box and choose your colour manually. Thety are useful, however, if you want to keep your colour consistent across your app. You can cut and paste the values whenever you want.
Tags: Adjusting the colour of elements, AppBuilder, Building your app, Creating Apps