nanorc color limitations

According the nanorc man, «legal colors for foreground and background color are: white, black, red, blue, green, yellow, magenta, and cyan. You may use the prefix "bright" to force a stronger color highlight for the foreground. If your terminal supports transparency, not specifying a bgcolor tells nano to attempt to use a transparent background.»

So, there's no way to use other colors rather than white, black, red, blue, green, yellow, magenta, and cyan? Other colours under the X11 color names, or something?

Why are (just) these colors used?

Thanks.