Joe's own editor: changing syntax highlighting

To be very specific and change the dark blue on black highlighting for PHP scripts in Joe:

In Ubuntu 16.04 and with Joe 4.1-2, edit:

/usr/share/joe/syntax/php.jsf

And change:

=Constant_sq blue

To some other color of your choosing:

=Constant_sq yellow

As mentioned you'll need to edit the appropriate .jsf file for your scripting language of choice, and the affected variable. To fix XML recoloring, edit xml.jsf and change:

=Tag blue

to

=Tag yellow.

Coloring files could also reside in /etc/joe/syntax/, just copy appropriate file to ~/.joe/syntax/ and edit it.