Can I type the exact hex code of my brand colour?

Yes. On Settings → Appearance → Color Settings, every colour has a swatch and a text box beside it. Type the hex code, such as #1A73E8, straight into the box.

The colours you can set are Primary, Secondary, Global, Base, Link, Link Hover, Info, Success, Warning, Alert, Divider and Dark. Put your main brand colour in Primary, since buttons and accents use it.

Click Save Changes when you're done. Your store's sections read these colours, so the change shows across the theme.