As you can read from :h 'termguicolors':
'termguicolors' 'tgc' boolean (default off)
Enables 24-bit RGB color in the TUI. Uses "gui" :highlight
attributes instead of "cterm" attributes.
So your highlighting command can not work since it doesn't provide a guifg attribute. I think the following should do the trick:
You have mistaken your terminal color scheme with the Vim color theme. To change the Vim color theme, you need to use :color command, followed by a valid color theme on your system, for example, color desert.