The themes are specified with RGB colors, and fish tries to pick the closest color supported by the terminal. It can do the 14 ANSI colors and the xterm 256 colors. fish 2.2 adds support for 24 bit color, which a few terminals can do (Konsole, iTerm2).

fish has a set_color builtin which accepts RGB hex. This makes colored output easy: `set_color 69F; echo 'I am light purple!'`