In the interests of helping someone else solve the same problem, here’s how I did it, the Quark AppleScript reference is wrong:
make character spec at end with properties {name:"hello world 3", «class CATR»:{base shift:"0 pt", color:color spec "5th Black Film", font:"Gill Sans Bk", horizontal scale:"100%", shade:"100%", size:"6.75 pt", style:{class:text style info, on styles:{plain}, off styles:{bold, italic, underline, outline, shadow, superscript, subscript, superior, strikethrough, all caps, small caps, word underline}}, track:"0", vertical scale:"100%"}}
One oddity i found was that you could have either:
color:color spec "5th Black Film"
or:
color:"5th Black Film"
But the named colour must exist
The only part of this I could not get to work is “base style”, which would usually go like this:
make character spec at end with properties {name:"hello world 3", base style: character spec "specName", «class CATR»:{...}}
But I can’t get that to work, just get a “can’t make a character spec” error.
Any clues on setting the base style property very much appreciated.