You can add in a promo sale banner that appears on all pages on your website.
Login to your website, go to homepage, edit page, add in a new block to Sale Banner area. Fill in details for Sale Banner as appropriate.
Colour Picker: You need an HTML colour reference, hexadecimal value. Online tool to help:
If you cannot find the Sale Banner block, it may not have been installed yet.
In which case, login to your website, search for "block types" then click "install" next to Grayline Sale Banner block.
Once this is done, follow the steps above.