[NTG-context] Customising referencing failures (\in, etc)

Folks, Is there a way to customise the ?? that is substituted in the output when a reference does not work out (nonexistent reference label, etc)? I’d like to make it more obvious for proofreading (like making it red, putting a marker in the margin, or something). Cheers, Robin \starttext \input tufte Bad reference \in[missing]. \input tufte \stoptext