Terminal even offers direct access to over 16 million colors, this is called "true color" mode. If the changes you make to the palette do not seem to have an effect, presumably the contents you see consist of such extended palette colors or true colors, rather than the 16 base colors. Gogh is a set of Bash scripts that makes it easy to change the color scheme of terminals in Linux and macOS].
Currently, it offers 190 terminal color schemes and supports Gtk-based terminals, such as Gnome Terminal, Xfce Terminal, Mate Terminal, Pantheon Terminal, Tilix and Guake on Linux and iTerm2 on the Mac. Color Scheme for Gnome Terminal, Pantheon Terminal, Tilix, and XFCE4 Terminal Color Schemes For Ubuntu, Linux Mint, Elementary OS and all distributions that use Gnome Terminal, Pantheon Terminal, Tilix, or XFCE4 Terminal; initially inspired by Elementary OS Luna. Also works on iTerm for macOS.
You can check out the themes here. As noted at the github page, In Gnome terminal, you can add or edit profiles from the menu bar. However, this functionality is not easliy available from command line.
Here, you'll find a script that will set the palette colors, foreground, background and highlight colors to a light or dark color scheme, overwriting a color profile you choose. Gogh is a collection of color schemes for various terminal emulators, including Gnome Terminal, Pantheon Terminal, Tilix, and XFCE4 Terminal. These schemes are designed to make your terminal more visually appealing and improve your productivity by providing a better contrast and color differentiation.
On gnome-terminal's UI you can only configure the first 16 of these. That is, probably the blue and green colors your ls produces are simply not the "standard" blue/green colors, but one of the "extended" ones. This article dives deep into the colours, formatting and customisation of gnome-terminal, the default bash terminal for Ubuntu.
The majority of this article applies to many terminal variants, not only to Gnome/Ubuntu. Inside each profile folder there's the palette key which is used to set the 16 ANSI terminal colors, and there are more specific ones like background-color, foreground-color, cursor-background-color and so on. You could even write scripts using gsettings or dconf to automate setting color palettes, which is what Gogh does.
Terminal Palette Test Page This is a tool to help visualize and test the contrast of terminal palettes with various common terminal applications that make use of colors. Things that are not yet done on this page: More applications! What have you seen on the terminal that uses color? Lets get it added. Custom palettes? Most popular software terminal applications, including GNOME, KDE, and Xfce, ship with the option to change their color theme.
Adjusting your theme is as easy as adjusting application preferences.