Can I/how to specify colors in hex or RGB in nano syntax highlight config?

I enabled syntax highlight in nano (PHP), but not happy with the default, I would like for example to have the comments displayed in very light grey. However, the documentation I found seems to suggest I can only write colors like "yellow", "red" etc. Is there a way to specify a color by its hex/RGB code? Is there a limitation in the number of colors bash/nano and so on can display? Obviously I am not very experienced with the Linux world. Same question for VIM, I might switch to VIM if that is not possible.